MIPS: Repair Kbuild make clean breakage.

When running make clean, Kbuild doesn't process the .config file, so nothing
generates a platform-y variable.  We can get it to descend into the platform
directories by setting $(obj-).

The dec Platform file was unconditionally setting platform-, obliterating
its previous contents and preventing some directories from being cleaned.
This is change to an append operation '+=' to allow cavium-octeon to be
cleaned.
Signed-off-by: David Daney <ddaney@caviumnetworks.com>
Cc: Sam Ravnborg <sam@ravnborg.org>
Patchwork: https://patchwork.linux-mips.org/patch/1718/Signed-off-by: Ralf Baechle <ralf@linux-mips.org>

arch/mips/Kbuild

@@ -7,6 +7,10 @@ subdir-ccflags-y := -Werror
 include arch/mips/Kbuild.platforms
 obj-y := $(platform-y)
 
+# make clean traverses $(obj-) without having included .config, so
+# everything ends up here
+obj- := $(platform-)
+
 # mips object files
 # The object files are linked as core-y files would be linked
 

arch/mips/dec/Platform

 #
 # DECstation family
 #
-platform-$(CONFIG_MACH_DECSTATION)	= dec/
+platform-$(CONFIG_MACH_DECSTATION)	+= dec/
 cflags-$(CONFIG_MACH_DECSTATION)	+= \
 			-I$(srctree)/arch/mips/include/asm/mach-dec
 libs-$(CONFIG_MACH_DECSTATION)		+= arch/mips/dec/prom/