test: capsule: Modify the capsule tests to use GUID values for sandbox

test: capsule: Modify the capsule tests to use GUID values for sandbox

The current UEFI capsule updation code uses two GUID values, one for
FIT images, and one for raw images across platforms. This logic is
being changed to have GUID values per image, per platform. Change the
tests for the capsule update code to reflect this change. The GUID
values now used are the ones specific to the sandbox platform -- one
for the u-boot image, and another for the u-boot environment image.

Install the FMP instance for raw images on the sandbox variant for
testing the capsule update code. Install the FMP instance for the FIT
images on the sandbox64 and sandbox_flattree variant for testing
capsule update for FIT images. This is being done by splitting the
capsule update script for FIT and raw images.

Signed-off-by: Sughosh Ganu <sughosh.ganu@linaro.org>