SPM: Assert value of `ENABLE_SPM` build flag

The Makefile was missing a check to verify that the value of
`ENABLE_SPM` is boolean.

Change-Id: I97222e4df9ae2fbd89cdb3263956dca52d360993
Signed-off-by: Antonio Nino Diaz <antonio.ninodiaz@arm.com>

Makefile

@@ -497,6 +497,7 @@ $(eval $(call assert_boolean,ENABLE_PMF))
 $(eval $(call assert_boolean,ENABLE_PSCI_STAT))
 $(eval $(call assert_boolean,ENABLE_RUNTIME_INSTRUMENTATION))
 $(eval $(call assert_boolean,ENABLE_SPE_FOR_LOWER_ELS))
+$(eval $(call assert_boolean,ENABLE_SPM))
 $(eval $(call assert_boolean,ENABLE_SVE_FOR_NS))
 $(eval $(call assert_boolean,ERROR_DEPRECATED))
 $(eval $(call assert_boolean,GENERATE_COT))
@@ -541,6 +542,7 @@ $(eval $(call add_define,ENABLE_PMF))
 $(eval $(call add_define,ENABLE_PSCI_STAT))
 $(eval $(call add_define,ENABLE_RUNTIME_INSTRUMENTATION))
 $(eval $(call add_define,ENABLE_SPE_FOR_LOWER_ELS))
+$(eval $(call add_define,ENABLE_SPM))
 $(eval $(call add_define,ENABLE_SVE_FOR_NS))
 $(eval $(call add_define,ERROR_DEPRECATED))
 $(eval $(call add_define,GICV2_G0_FOR_EL3))
@@ -555,7 +557,6 @@ $(eval $(call add_define,PROGRAMMABLE_RESET_ADDRESS))
 $(eval $(call add_define,PSCI_EXTENDED_STATE_ID))
 $(eval $(call add_define,RESET_TO_BL31))
 $(eval $(call add_define,SEPARATE_CODE_AND_RODATA))
-$(eval $(call add_define,ENABLE_SPM))
 $(eval $(call add_define,SPD_${SPD}))
 $(eval $(call add_define,SPIN_ON_BL1_EXIT))
 $(eval $(call add_define,TRUSTED_BOARD_BOOT))