scripts/setlocalversion: fix a bug when LOCALVERSION is empty

The commit 042da426f8eb ("scripts/setlocalversion: simplify the short
version part") reduces indentation. Unfortunately, it also changes behavior
in a subtle way - if the user has empty "LOCALVERSION" variable, the plus
sign is appended to the kernel version. It wasn't appended before.

This patch reverts to the old behavior - we append the plus sign only if
the LOCALVERSION variable is not set.

Fixes: 042da426f8eb ("scripts/setlocalversion: simplify the short version part")
Signed-off-by: Mikulas Patocka <[e-mail]>
Signed-off-by: Masahiro Yamada <[e-mail]>

scripts/setlocalversion

···
 if test "$CONFIG_LOCALVERSION_AUTO" = "y"; then
 	# full scm version string
 	res="$res$(scm_version)"
-elif [ -z "${LOCALVERSION}" ]; then
-	# append a plus sign if the repository is not in a clean
-	# annotated or signed tagged state (as git describe only
-	# looks at signed or annotated tags - git tag -a/-s) and
-	# LOCALVERSION= is not specified
+elif [ "${LOCALVERSION+set}" != "set" ]; then
+	# If the variable LOCALVERSION is not set, append a plus
+	# sign if the repository is not in a clean annotated or
+	# signed tagged state (as git describe only looks at signed
+	# or annotated tags - git tag -a/-s).
+	#
+	# If the variable LOCALVERSION is set (including being set
+	# to an empty string), we don't want to append a plus sign.
 	scm=$(scm_version --short)
 	res="$res${scm:++}"
 fi