fs/ext2: use a post-gen hook rather than a post-target rule

post-target rules are probably not resilient to parallel builds, given
that they do not depend on the image being generated first.

Beside, we already have a mechanism for running stuff after the
filesystem is generated, and that's called post-gen hooks.

Use those hooks.

Note: this basically reverts 75b6303 (rootfs-ext2: make the symlink as a
_POST_TARGET) since we've now re-introduced post-gen hooks.

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Cc: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Cc: Arnout Vandecappelle <arnout@mind.be>
Reviewed-by: Romain Naour <romain.naour@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>

fs/ext2/ext2.mk

@@ -36,13 +36,11 @@ define ROOTFS_EXT2_CMD
 	}
 endef
 
-rootfs-ext2-symlink:
-	ln -sf rootfs.ext2$(ROOTFS_EXT2_COMPRESS_EXT) $(BINARIES_DIR)/rootfs.ext$(BR2_TARGET_ROOTFS_EXT2_GEN)$(ROOTFS_EXT2_COMPRESS_EXT)
-
-.PHONY: rootfs-ext2-symlink
-
 ifneq ($(BR2_TARGET_ROOTFS_EXT2_GEN),2)
-ROOTFS_EXT2_POST_TARGETS += rootfs-ext2-symlink
+define ROOTFS_EXT2_SYMLINK
+	ln -sf rootfs.ext2$(ROOTFS_EXT2_COMPRESS_EXT) $(BINARIES_DIR)/rootfs.ext$(BR2_TARGET_ROOTFS_EXT2_GEN)$(ROOTFS_EXT2_COMPRESS_EXT)
+endef
+ROOTFS_EXT2_POST_GEN_HOOKS += ROOTFS_EXT2_SYMLINK
 endif
 
 $(eval $(call ROOTFS_TARGET,ext2))