fix(build): ensure `$(ROT_KEY)` depends on correct directory rules

In order for directories to be automatically created when used as a
dependency, they must end with a forward slash (`/`). This is because we
have a pattern rule (`%/`) to create a directory anywhere where a
directory is required as a direct dependency.

Change-Id: Ib632d59da0745f6cadb0a839a62360aeca25c178
Signed-off-by: Chris Kay <chris.kay@arm.com>
10 files changed