Rebase build scripts and variant files

Rebase the build scripts to the latest scritps from ARM.

Since starting to use these scripts, they diverged quite a bit, so this
is a squashed commit use to catch up both the ARM tree and the ARMLT
tree.

Change-Id: Ifbaf62fb0168852f7579f82dc87178554c415349
Signed-off-by: James King <james.king@arm.com>