firmware: Simplify directory creation

When building the firmware blobs, use a simple loop to create
directories in $(objtree), like in Makefile.build. This simplifies the
rules and also makes it possible to set $(objtree) to '.' later. Before
this change, a dependency on $(objtree)/<dir> would be satisfied by
<dir> in $(srctree).

When installing the firmware blobs, call mkdir like in Makefile.modinst.

Cc: David Woodhouse <dwmw2@infradead.org>
Acked-by: Sam Ravnborg <sam@ravnborg.org>
Signed-off-by: Michal Marek <mmarek@suse.cz>