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>
1 file changed