build-uefi: allow build scripts to add UEFI build parameters

Allow build scripts to add additional build configurations. This allows
build scripts to add build parameters during the build process. These
build parameters should not be included in the platform build config
file but instead created and exported during the build process.

Change-Id: I3404c3c44de0f12015e943eeb708fc7e68b4a478
Signed-off-by: Jagadeesh Ujja <jagadeesh.ujja@arm.com>