kexec-tools: fix recompile error

kexec-tools: fix recompile error

Fixed:
NOTE: make -j 32
make: *** No rule to make target `/path/to/sysroot/4.9.0/include/stddef.h', needed by `kexec/kexec.o'.  Stop.

This happens when upgrade gcc from 4.9.0 to 4.9.1, and the kexec/kexec.d
isn't regenerated when recompile, the content of it are:

[snip]
kexec/kexec.o: /path/to/sysroot/4.9.0/include/stddef.h
[snip]

And Makefile includes the kexec/kexec.d file if it exists, so there
would be errors when /path/to/sysroot/4.9.0/include/stddef.h doesn't
exist.

Remove kexec/kexec.d and other similar files will fix the problem.

Signed-off-by: Robert Yang <liezhi.yang@windriver.com>
Signed-off-by: Saul Wold <sgw@linux.intel.com>