Allow $DRACUT_INSTALL to be not an absolute path

Allow $DRACUT_INSTALL to be not an absolute path

/usr/sbin/dracut-install is a symlink:
$ file /usr/sbin/dracut-install
/usr/sbin/dracut-install: symbolic link to ../lib/dracut/dracut-install

It resulted to "dracut-install not found" error:

+++ for p in $DRACUT_PATH
+++ [[ -L /sbin/dracut-install ]]
+++ [[ -x /sbin/dracut-install ]]
+++ for p in $DRACUT_PATH
+++ [[ -L /bin/dracut-install ]]
+++ [[ -x /bin/dracut-install ]]
+++ for p in $DRACUT_PATH
+++ [[ -L /usr/sbin/dracut-install ]]
+++ printf '%s\n' dracut-install
+++ return 0
++ DRACUT_INSTALL=dracut-install
++ [[ -n dracut-install ]]
++ [[ -n dracut-install ]]
++ DRINSTALLPARTS=0
++ for i in $DRACUT_INSTALL
++ DRINSTALLPARTS=1
++ [[ 1 = 1 ]]
++ [[ -x dracut-install ]]
++ dfatal 'dracut-install not found!'
++ set +x
dracut: dracut-install not found!

[ -x <not a path to file> ] is not correct and will always be false.
But actually it is available. Let's just allow it to be not an absolute path.
Maybe some other places can be improved to avoid DRACUT_INSTALL being not an absolute path.

Signed-off-by: Mikhail Novosyolov <m.novosyolov@rosalinux.ru>