gcc-cross-initial: Ensure it uses an isolated sysroot
If we don't do this, a stale limits.h may be detected in STAGING_DIR_TARGET
which would result in a different limits.h getting generated by gcc-cross-initial
that references it. The referenced limits.h will then not get found by eglibc-initial
causing rather strange build failures.
The simplest solution is to create a temporary sysroot containing only the things
gcc-cross-initial should care about and this results in a correct limits.h file
regardless of what else may have been built.
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
