lib.sh: stricter check in chkvars()

testing +x is all well and good, but the variable string
may be empty, even if set. some of the checks in the build
system are relying on the latter, so handle it.

Signed-off-by: Leah Rowe <leah@libreboot.org>

include/lib.sh

@@ -33,6 +33,7 @@ chkvars()
 {
 	for var in $@; do
 		eval "[ -n "\${$var+x}" ] || \$err \"$var unset\""
+		eval "[ -n "\$$var" ] || \$err \"$var unset\""
 	done; return 0
 }