overlay: move the ebuilds to their own repo, pin it in overlay.conf

[OctopusET]

Stacked on provider-buildroot.

• overlay: move the ebuilds to their own repo, pin it in overlay.conf
• docs: buildroot rootfs provider
• boards/qemu-riscv64-buildroot: buildroot provider example
• provider: buildroot rootfs from a defconfig
• docs: fedora rootfs provider, and what fedora publishes for riscv64
• provider: install fedora packages with dnf5 from the overlay
• provider: fedora rootfs from a pinned container base image
• overlay: add app-arch/apk-tools, emerge it instead of apk.static
• docs: alpine rootfs provider, and why fedora is not one
• boards/qemu-riscv64-alpine: alpine provider example for qemu virt
• provider: alpine rootfs via static apk, no emulation
• docs: ubuntu provider and ROOTFS_SECOND_STAGE
• boards/qemu-riscv64-ubuntu: ubuntu provider example
• provider: ubuntu rootfs and opt-in first-boot second stage
• target,image: merged-usr profile link, board package.provided
• portage: shell-quote every emerge atom against bash globbing
• container: preserve /dev as empty dir in stage3 unpack/pack
• cli: respect --arch when resolving fallback target
• target: make musl rv32 stage1 actually work
• crossdev-stages: add musl rv32 support
• fix(sandbox): create the overlay dir through the container, not the host
• verify: a check that examined nothing must say so, not pass
• image: write cross-prefix keywords inside the container, not under the overlay
• update: bring the sandbox packages and a board binpkg cache up to date
• assemble: drop workspace markers from the image rootfs
• docs: debian rootfs provider
• boards/qemu-riscv64-debian: debian provider example for qemu virt
• provider: debian rootfs via debootstrap
• docs: ROOTFS_PROVIDER in board.conf table and design
• image: route provisioning, deps, and os-config through the provider
• image: extract OpenRC os-config from default_assemble
• board: ROOTFS_PROVIDER selects who fills the image rootfs
• toolchain: give cargo the triple rustc knows, not the CHOST
• toolchain: add ToolchainEnv for in-sandbox cross-compilation
• container: with_env on SandboxRunner
• workspace: add Workspace::at for a non-XDG cache root
• lib: expose crate as crossdev_stages library
• image: run the ABI check in the build, and stop the menu saying Gentoo
• chroot: open a shell inside the board rootfs through qemu-user
• readme: --board on target quickstart, BOARD_GCC_VERSION, pin semantics
• sandbox: refresh the portage overlay on every prepare
• target: quote gcc pin atom against shell glob expansion
• overlay: make ESOS ebuilds actually installable
• portage: gcc/llvm version pins + scoped PORTAGE_TMPDIR
• portage: MAKEOPTS load-average + parallel-fetch + tmpfs build dir
• target update: pin crossdev gcc to board, --noreplace, explicit jobs
• overlay: ship defaults/overlay/ with ESOS firmware ebuilds
• store: say what produced a binary package cache, and when it moved
• cflags: key a board cache by its workarounds too, not just BOARD_CFLAGS
• verify: check the image can load the symbol versions it asks for
• board: label boards by arch, SoC and vendor with TAGS
• readme: match the kernel tag and CFLAGS each board actually sets
• image: refuse to ship a binary the board cannot execute
• bootloader: fail when a stage has a repo but no ref to clone
• gitignore: put the evidence exception after the rule it has to beat
• cflags: refuse flags that cannot key a binary package cache
• enter: open a shell in the container a board build runs in
• image: run the ISA check when assemble finishes
• verify: check a board binaries against the ISA its CFLAGS promise
• boards: give the SpacemiT K1 boards a shared include
• image: rebuild the image tree each assemble instead of merging into it
• image: copy the firmware a board asks for, and fail when it is missing
• image: write the extlinux config instead of every board repeating it
• board: keep the default build steps in one place, not in every board
• image: disable the gettys baselayout starts on ports a board lacks
• boards: give the RK35xx boards a shared include
• boards/odroid-m1,m1s,m2: boot by PARTUUID and stop claiming an ESP
• boards: pin the DDR blobs and restore WERROR=0 on k3
• boards: name images after what you do with them, not after a guess
• board: let a board include shared config chunks