kexecboot: fix build when S != B

* fatal error: ../config.h: No such file or directory

Signed-off-by: Andrea Adami <andrea.adami@gmail.com>
kexecboot: fix configure warnings

Makefile.am:5: warning: source file 'fstype/fstype.c' is in a subdirectory,
Makefile.am:5: but option 'subdir-objects' is disabled
automake: warning: possible forward-incompatibility.
...
Makefile.am:5: warning: source file 'machine/zaurus.c' is in a subdirectory,
...

Signed-off-by: Andrea Adami <andrea.adami@gmail.com>