The PLATFORM configuration option is not used in any unique way. With simple
changes, we can directly use CONFIG_PLATFORM_UEFI and CONFIG_PLATFORM_SBL to
compile the ACRN hypervisor. This patch removes this config option and adjusts
the hypervisor Makefile accordingly.
The "old" way of specifying the platform from the command-line is not affected
by these changes, e.g.: "make PLATFORM=sbl"
Tracked-On: #1588
Signed-off-by: Geoffroy Van Cutsem <geoffroy.vancutsem@intel.com>