efi/libstub: Refactor the cmd_stubcopy Makefile command

It took me a while to understand what is going on in the nested
if-blocks.

Simplify it by removing unneeded code.

  - if_changed automatically adds 'set -e', so any failure in the
    series of commands makes it immediately fail as a whole.
    So, the outer if block is entirely redundant.

  - Since commit 9c2af1c7 ("kbuild: add .DELETE_ON_ERROR special target"),
    GNU Make automatically deletes the target on any failure
    in its recipe. The explicit 'rm -f $@' is redundant.

  - Surrounding commands with ( ) will spawn a subshell to execute them
    in it, but it is rarely useful to do so.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Matt Fleming <matt@codeblueprint.co.uk>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: linux-efi@vger.kernel.org
Link: http://lkml.kernel.org/r/20190328193429.21373-2-ard.biesheuvel@linaro.org


Signed-off-by: Ingo Molnar <mingo@kernel.org>