Fix `--ignore-missing` in Mconfig depfiles
Fix `--ignore-missing` in Mconfig depfiles

This commit fixes a bug where a nonexistent `Mconfig` would still be
added to the depfile despite `--ignore-missing` option being set.

Change-Id: I6be7aeab96c9570d7c453a327901e684072c3cff
Signed-off-by: Alexander Khabarov <410973c8966e1f78eb22e25cb3609e3bbce9769a@arm.com>

ARM-software/bob-build,ARM-software/bob-build,ARM-software/bob-build,ARM-software/bob-build,ARM-software/bob-build