fs/ext2: fix typo

BR2_TARGET_ROOTFS_EXT2_RESBLKS is an int, so test against 0 rather than
the empty string - Otherwise the test is always true and a -m option
without arguments is added to the argument list, causing genext2fs to
get confused.

Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>

fs/ext2/ext2.mk

@@ -18,7 +18,7 @@ ifneq ($(strip $(BR2_TARGET_ROOTFS_EXT2_INODES)),0)
 EXT2_OPTS += -N $(BR2_TARGET_ROOTFS_EXT2_INODES)
 endif
 
-ifneq ($(strip $(BR2_TARGET_ROOTFS_EXT2_RESBLKS)),)
+ifneq ($(strip $(BR2_TARGET_ROOTFS_EXT2_RESBLKS)),0)
 EXT2_OPTS += -m $(BR2_TARGET_ROOTFS_EXT2_RESBLKS)
 endif