MODSIGN: Avoid using .incbin in C source

Using the asm .incbin statement in C sources breaks any gcc wrapper which
assumes that preprocessed C source is self-contained. Use a separate .S
file to include the siging key and certificate.

[ This means we no longer need SYMBOL_PREFIX which is defined in kernel.h
  from cbdbf2abb7844548a7d7a6a2ae7af6b6fbcea401, so I removed it -- RR ]

Tested-by: Michal Marek <mmarek@suse.cz>
Signed-off-by: Takashi Iwai <tiwai@suse.de>
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
Acked-by: James Hogan <james.hogan@imgtec.com>