Don't store generated firmware object files in the source directory.

Don't store generated firmware object files in the source directory.

Trim the leading directory of a firmware source file from the resulting
target object file name so the object file is stored in the object
directory. Previously, using 'FIRMWS= /path/to/fw.bin:fw.bin' would
store the generated 'fw.bin.fwo' file in the /path/to directory. Now
it stores it in the object directory of the kernel module being built.

Reviewed by:	bdrewery
MFC after:	1 week
Differential Revision:	https://reviews.freebsd.org/D6285