kexecboot.c: workaround for absolute kernel and initrd symlinks

Add MOUNTPOINT prefix if the kernel/initrd symlinks start with '/'.
Do nothing if the path is a relative symbolic link or not a symlink.

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>