kbuild: Fix reading of .config in link-vmlinux.sh

The shell '.' command is not required to search the current directory as
a fallback and in fact newer versions of bash in sh-mode do not do this.
Force reading the file from the current directory if $KCONFIG_CONFIG
contains no '/'.
Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: Michal Marek <mmarek@suse.cz>

scripts/link-vmlinux.sh

@@ -132,7 +132,14 @@ if [ "$1" = "clean" ]; then
 fi
 
 # We need access to CONFIG_ symbols
-. ${KCONFIG_CONFIG}
+case "${KCONFIG_CONFIG}" in
+*/*)
+	. "${KCONFIG_CONFIG}"
+	;;
+*)
+	# Force using a file from the current directory
+	. "./${KCONFIG_CONFIG}"
+esac
 
 #link vmlinux.o
 info LD vmlinux.o