genromfs: use $(HOST_DIR) instead of $(HOST_DIR)/usr as prefix

genromfs is special because it uses "PREFIX" in the meaning of DESTDIR
and "prefix" in the meaning of prefix. We were up to know using it
incorrectly for host: PREFIX shouldn't be set and only prefix should
be set.

Add an explanatory comment for this unusual behaviour.

Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>

package/genromfs/genromfs.mk

@@ -16,6 +16,8 @@ define GENROMFS_BUILD_CMDS
 		LDFLAGS="$(TARGET_LDFLAGS)"
 endef
 
+# "PREFIX" is the equivalent of DESTDIR in autotools, "prefix" is the
+# traditional prefix. "prefix" defaults to /usr so no need to set it.
 define GENROMFS_INSTALL_TARGET_CMDS
 	$(TARGET_MAKE_ENV) $(MAKE) -C $(@D) PREFIX=$(TARGET_DIR) install
 endef
@@ -25,7 +27,7 @@ define HOST_GENROMFS_BUILD_CMDS
 endef
 
 define HOST_GENROMFS_INSTALL_CMDS
-	$(HOST_MAKE_ENV) $(MAKE) -C $(@D) PREFIX=$(HOST_DIR) install
+	$(HOST_MAKE_ENV) $(MAKE) -C $(@D) prefix=$(HOST_DIR) install
 endef
 
 $(eval $(generic-package))