common.mk: tweak rules to workaround make-3.80 bugs

I got some reports from people who use make-3.80 that mtd-utils wasn't
building correctly the first time.  Turns out that older versions of make
misbehave with pattern rules and full paths.  So I've tweaked the code a
little to work with make-3.80 and make-3.81 (the latest release).

Also, I added a small optimization to avoid running `mkdir` when building
in-tree.

Signed-off-by: Mike Frysinger <vapier@gentoo.org>
Signed-off-by: Artem Bityutskiy <Artem.Bityutskiy@nokia.com>

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