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>
Use /boot as default MOUNTPOINT

The symlink pointing to the kernel in /boot are now absolute so we
have to follow the behavior of OpenEmbedded and some major distros.

Fix following situation:

root@mizar:/var/tmp# ls -al boot/
total 2076
drwxr-xr-x 2 root root    4096 lug  5 01:38 .
drwxrwxrwt 4 root root    4096 lug  5 12:26 ..
-rw-r--r-- 1 root root     831 lug  5 01:24 boot.cfg
-rw-r--r-- 1 root root    1322 lug  5 01:24 icon.xpm
lrwxrwxrwx 1 root root      34 lug  5 12:26 zImage ->
/boot/zImage-3.14.5-yocto-standard
-rw-r--r-- 1 root root 2106832 lug  5 01:20 zImage-3.14.5-yocto-standard

Cannot open `/mnt/boot/zImage': No such file or directory
Nothing has been loaded!

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