ARM: 7120/1: remove bashism in check for multiple zreladdrs

Get rid of this complaint from dash:

    AS      arch/arm/boot/compressed/lib1funcs.o
  /bin/sh: 1: [: y: unexpected operator
    LD      arch/arm/boot/compressed/vmlinux
Acked-by: Sascha Hauer <s.hauer@pengutronix.de>
Signed-off-by: Rabin Vincent <rabin@rab.in>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>

arch/arm/boot/compressed/Makefile

@@ -140,7 +140,7 @@ bad_syms=$$($(CROSS_COMPILE)nm $@ | sed -n 's/^.\{8\} [bc] \(.*\)/\1/p') && \
     echo "$$bad_syms" >&2; rm -f $@; false )
 
 check_for_multiple_zreladdr = \
-if [ $(words $(ZRELADDR)) -gt 1 -a "$(CONFIG_AUTO_ZRELADDR)" == "" ]; then \
+if [ $(words $(ZRELADDR)) -gt 1 -a "$(CONFIG_AUTO_ZRELADDR)" = "" ]; then \
 	echo 'multiple zreladdrs: $(ZRELADDR)'; \
 	echo 'This needs CONFIG_AUTO_ZRELADDR to be set'; \
 	false; \