SPMD: add command line parameter to run SPM at S-EL2 or S-EL1

Added SPMD_SPM_AT_SEL2 build command line parameter.
Set to 1 to run SPM at S-EL2.
Set to 0 to run SPM at S-EL1 (pre-v8.4 or S-EL2 is disabled).
Removed runtime EL from SPM core manifest.

Change-Id: Icb4f5ea4c800f266880db1d410d63fe27a1171c0
Signed-off-by: Artsem Artsemenka <artsem.artsemenka@arm.com>
Signed-off-by: Max Shvetsov <maksims.svecovs@arm.com>