bitbake.conf: add default for IMAGE_FSTYPES_DEBUGFS

If debug filesystem generation is enabled but this isn't assigned then the
generation code throws exceptions.

(From OE-Core rev: 0a1b02fab0e2604cd55ea6f45d764a864599213a)

Signed-off-by: Ross Burton <ross.burton@intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

meta/conf/bitbake.conf

@@ -713,6 +713,7 @@ require conf/sanity.conf
 DL_DIR ?= "${TOPDIR}/downloads"
 SSTATE_DIR ?= "${TOPDIR}/sstate-cache"
 IMAGE_FSTYPES ?= "tar.gz"
+IMAGE_FSTYPES_DEBUGFS ?= "${IMAGE_FSTYPES}"
 
 INITRAMFS_FSTYPES ?= "cpio.gz"
 # The maximum size in Kbytes for the generated initramfs image size.