The portage CBUILD and HOSTCC variables need to be set to the SDK host to get
a proper cross build when building target binaries.
Change _configure_sysroot to use the CBUILD environment variable to set the
CBUILD and HOSTCC variables of ${ROOT}/etc/portage/make.conf. Also, fix up all
calls to _configure_sysroot to set the CBUILD environment variable.
Fixes setup_board failure when the host and target architectures differ.
Signed-off-by: Geoff Levand <geoff@infradead.org>
[marineam: fixed a copy/paste error]