Platform: Disable IRQs from startup files

This patch removes the disabling of IRQs from MCUBoot and adds it to
the startup code of secure code for supported platforms (AN519, AN521
and Musca A1). This way when the bootloader passes execution to runtime
firmware, the value of PRIMASK register will be the same as after reset.

Change-Id: I4538dc92d2dd307362a5632481e2e23c584ed708
Signed-off-by: David Vincze <david.vincze@arm.com>
7 files changed