EAS test kernel support for all variants.

If the linux kernel config fragments for EAS are present, use them.
Also let the user provide BL_SUPPORT=EAS or BL_SUPPORT=big-LITTLE-MP
variable to the build scripts if desired.

Will warn and try to provide help if your chosen config does not exist
in the kernel source.

Signed-off-by: Chris Redpath <chris.redpath@arm.com>