video/console: fix spurious rebuild

kbuild does have troubles with assignmnets including '#'.
The '#' is seen as a comment marker and this will in the end cause
kbuild to think the commandline to build promcon_tbl.c has changed.
This happens because the commandline is stored in the file: .promcon_tbl.c.cmd

Although a bit complex the command to build promcon_tbl.c is unlikely
to change so the workaround is to skip the check for a changed commandline.
Now promcon_tbl.c is only rebuilt if the .uni file is newer than the .c file.
Signed-off-by: Sam Ravnborg <sam@ravnborg.org>

drivers/video/console/Makefile

@@ -37,5 +37,5 @@ quiet_cmd_conmakehash = CNMKHSH $@
 		sed -e '/\#include <[^>]*>/p' -e 's/types/init/' \
 		-e 's/dfont\(_uni.*\]\)/promfont\1 __initdata/' > $@
 
-$(obj)/promcon_tbl.c: $(src)/prom.uni FORCE
-	$(call if_changed,conmakehash)
+$(obj)/promcon_tbl.c: $(src)/prom.uni
+	$(call cmd,conmakehash)