firmware: Create directories for external firmware

Commit 5180d5f4

 ("firmware: Simplify directory creation") broke
including firmware specified in CONFIG_EXTRA_FIRMWARE:

  MK_FW   firmware/amd-ucode/microcode_amd.bin.gen.S
/bin/sh: firmware/amd-ucode/microcode_amd.bin.gen.S: No such file or directory
...
firmware/Makefile:185: recipe for target
'firmware/amd-ucode/microcode_amd.bin.gen.S' failed

It works with O= builds, because the directory is created by
Makefile.build. Create the directory in firmware/Makefile in non-O
builds.

Reported-and-tested-by: Ronald Uitermark <ronald645@gmail.com>
Reported-and-tested-by: Torsten Kaiser <just.for.lkml@googlemail.com>
Signed-off-by: Michal Marek <mmarek@suse.cz>