installer: fix staging error message when no firmware was built

Commit a4c8d4e ("Intel: remove XTOS support") stopped
installer/GNUmakefile from building all XTOS firmware by default:
nothing unless either SIGNED_list or UNSIGNED_list is passed on the
command line. This didn't break CI which sets one of these parameters.
But it resulted in a cryptic "test -e staging/sof" failure and lack of
error message when trying to build by default.

Add a one-line, explicit check. The comment doubles as an error message.

Signed-off-by: Marc Herbert <[email protected]>

Author: marc-hb

installer/GNUmakefile

@@ -75,6 +75,7 @@ STAGING_SOF_TPLG ?= staging/sof-tplg
 STAGING_TOOLS ?= staging/tools
 
 stage: signed unsigned ldicts aliases topologies tools
+	test -n "${UNSIGNED_list}${SIGNED_list}" # is there any FW to stage?
 ifneq (${STAGING_SOF_VERSION},${STAGING_SOF})
 	ln -sfT sof${VERSION_SUFFIX} ${STAGING_SOF}
 	test -e ${STAGING_SOF}