Since EAPI=7 was supported, portage can no longer use different
ROOT and SYSROOT values. This adjusts the paths so that the first
phase builds cross-toolchains under /usr/${CHOST}, then the native
toolchains are built under /build/${BOARD} (as was being done
previously). Now that the cross-toolchain development files can't
be used when building the native toolchain, the headers and libs
are stupidly copied into the board root to be used used and then
overwritten by the board packages as they are built. Since this is
all done in a chroot, these changes shouldn't affect the SDK host.
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