fix(arm): create build directory before key generation

Arm ROTPK generation may start before the build directory is
created, causing errors like:

 00:45:53.235 Can't open "/home/buildslave/workspace/tf-a-coverity/
 trusted-firmware-a/build/rd1ae/debug/arm_rotpk.bin" for writing,
 No such file or directory

This patch ensures the build directory is created beforehand to
prevent such issues.

Signed-off-by: Manish V Badarkhe <Manish.Badarkhe@arm.com>
Change-Id: I73f7d5af00efc738e95ea79c5cacecdb6a2d20c6
1 file changed