build: remove the `NM` variable

No part of the build system uses the `NM` variable, which is usually
used to dump symbol tables from compiled images. This change removes all
declarations of it.

Change-Id: I796ff365e6a7f97d21678f1c8cf8b59bfbb1ae9c
Signed-off-by: Chris Kay <chris.kay@arm.com>
2 files changed