modpost: Optionally ignore secondary errors seen if a single module build fails

Commit ea4054a2 (modpost: handle huge numbers of modules) added
support for building a large number of modules.

Unfortunately, the commit changed the semantics of the makefile: Instead of
passing only existing object files to modpost, make now passes all expected
object files. If make was started with option -i, this results in a modpost
error if a single file failed to build.

Example with the current btrfs build falure on m68k:

fs/btrfs/btrfs.o: No such file or directory
make[1]: [__modpost] Error 1 (ignored)

This error is followed by lots of errors such as:

m68k-linux-gcc: error: arch/m68k/emu/nfcon.mod.c: No such file or directory
m68k-linux-gcc: fatal error: no input files
compilation terminated.
make[1]: [arch/m68k/emu/nfcon.mod.o] Error 1 (ignored)

This doesn't matter much for normal builds, but it is annoying for builds
started with "make -i" due to the large number of secondary errors.
Those errors unnececessarily clog any error log and make it difficult
to find the real errors in the build.

Fix the problem by adding a new parameter '-n' to modpost. If this parameter
is specified, modpost reports but ignores missing object files.

With this patch, error output from above problem is (with make -i):

m68k-linux-ld: cannot find fs/btrfs/ioctl.o: No such file or directory
make[2]: [fs/btrfs/btrfs.o] Error 1 (ignored)
...
fs/btrfs/btrfs.o: No such file or directory (ignored)

Cc: Rusty Russell <rusty@rustcorp.com.au>
Cc: Michael Marek <mmarek@suse.cz>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>