ramips: define lzma loader platform in target

Loader platform is a per-soc variable instead of a per-device one.
Determine corresponding loader platform at the beginning of image
Makefile.

Signed-off-by: Chuanhong Guo <gch981213@gmail.com>

target/linux/ramips/image/Makefile

@@ -18,7 +18,11 @@ loadaddr-y := 0x80000000
 loadaddr-$(CONFIG_TARGET_ramips_rt288x) := 0x88000000
 loadaddr-$(CONFIG_TARGET_ramips_mt7621) := 0x80001000
 
+ldrplatform-y := ralink
+ldrplatform-$(CONFIG_TARGET_ramips_mt7621) := mt7621
+
 KERNEL_LOADADDR := $(loadaddr-y)
+LOADER_PLATFORM := $(ldrplatform-y)
 
 KERNEL_DTB = kernel-bin | append-dtb | lzma
 define Device/Default
@@ -68,7 +72,7 @@ define Build/loader-common
 	$(MAKE) -C lzma-loader \
 		PKG_BUILD_DIR="$@.src" \
 		TARGET_DIR="$(dir $@)" LOADER_NAME="$(notdir $@)" \
-		BOARD="$(BOARDNAME)" PLATFORM="$(PLATFORM)" \
+		BOARD="$(BOARDNAME)" PLATFORM="$(LOADER_PLATFORM)" \
 		LZMA_TEXT_START=0x82000000 LOADADDR=$(KERNEL_LOADADDR) \
 		$(1) compile loader.$(LOADER_TYPE)
 	mv "$@.$(LOADER_TYPE)" "$@"

target/linux/ramips/image/mt7621.mk

@@ -504,7 +504,6 @@ define Device/MikroTik
   IMAGE_SIZE := 16128k
   DEVICE_PACKAGES := kmod-usb3
   LOADER_TYPE := elf
-  PLATFORM := mt7621
   KERNEL := $(KERNEL_DTB) | loader-kernel
   IMAGE/sysupgrade.bin := append-kernel | kernel2minor -s 1024 | \
 	pad-to $$$$(BLOCKSIZE) | append-rootfs | pad-rootfs | append-metadata | \