vboot-utils: fix ARCH detection

The package includes some target-specific code that is irrelevant
in a host package but gets built anyway. The target for this code
must be one of the supported ChromeOS targets.

Supplied Makefile apparently relies on the environment to provide
a valid target, with a simple fallback to host arch. This breaks
the build if no value is provided and the host arch is not among
the supported ones.

Should fix
http://autobuild.buildroot.net/results/d118a83b6c4f7f910d0d44c279f36251d7ba29e8/


and similar failures.

Signed-off-by: Alex Suykov <alex.suykov@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>