build: Adds the ability to disable personal initramfs build for target device

If KERNEL_INITRAMFS := in the target/linux/*/image/Makefile->Device/%NAME% section is set to ''
then personal initramfs file for this target device will not be created.
This var is similar to the Device/Build/kernel KERNEL_INSTALL :=

Signed-off-by: Sergey Sergeev <adron@yapic.net>

include/image.mk

@@ -351,7 +351,7 @@ endef
 ifndef IB
 define Device/Build/initramfs
   $(call Device/Export,$(KDIR)/tmp/$$(KERNEL_INITRAMFS_IMAGE),$(1))
-  $$(_TARGET): $(BIN_DIR)/$$(KERNEL_INITRAMFS_IMAGE)
+  $$(_TARGET): $$(if $$(KERNEL_INITRAMFS),$(BIN_DIR)/$$(KERNEL_INITRAMFS_IMAGE))
 
   $(KDIR)/$$(KERNEL_INITRAMFS_NAME):: image_prepare
   $(BIN_DIR)/$$(KERNEL_INITRAMFS_IMAGE): $(KDIR)/tmp/$$(KERNEL_INITRAMFS_IMAGE)