crosssdk: Clear MACHINEOVERRIDES

crosssdk: Clear MACHINEOVERRIDES

Without this, things like arm* can make it into OVERRIDES when we're
building a compiler to build binaries for another architecture like
x86. This can can lead to build failures dependning on the
exact configuration and overrides.

For example:

MACHINE=imx53qsb bitbake gcc-crosssdk-initial-x86_64 -e | grep EXTRA_OECONF

was showing an armv7 configuration option to gcc.

(From OE-Core rev: 859ac3fdb75303f9f0b4bf1d8d83db0069f0a27b)

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>