package/mke2img: specifying zero inodes means auto

Currently, leaving the number of inodes to be autocalculated requires
the user to not specify the -i option at all.

Also accept zero as meaning auto; this will help simplify the ext2.mk
code a little bit.

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Samuel Martin <s.martin49@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>

package/mke2img/mke2img

@@ -59,7 +59,7 @@ main() {
     esac
 
     # calculate needed inodes
-    if [ -z "${nb_inodes}" ]; then
+    if [ ${nb_inodes:-0} -eq 0 ]; then
         nb_inodes=$(find "${root_dir}" | wc -l)
         nb_inodes=$((nb_inodes+400))
     fi