firmware: refactor firmware/Makefile

Clean up the Makefile. Equivalent *.gen.S files are still produced.

 - Use filechk to remove ugly wordsize_deps

 - Get FWNAME, FWSTR, ASM_WORD, ASM_ALIGN, and PROGBITS out of the
   recipe for readability

 - Remove 'mkdir' because filechk takes care of it

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>