kernel: Allow specifying a custom linker binary

* This is needed for using LLVM LLD ( and ThinLTO )

Change-Id: If603274326f0ff22f967455847ffe1c595f04aa5
diff --git a/build/tasks/kernel.mk b/build/tasks/kernel.mk
index ba70f29..b5690d0 100644
--- a/build/tasks/kernel.mk
+++ b/build/tasks/kernel.mk
@@ -43,6 +43,8 @@
 #
 #   KERNEL_CC                          = The C Compiler used. This is automatically set based
 #                                          on whether the clang version is set, optional.
+#   KERNEL_LD                          = The Linker used. This is automatically set based
+#                                          on whether the clang/gcc version is set, optional.
 #
 #   KERNEL_CLANG_TRIPLE                = Target triple for clang (e.g. aarch64-linux-gnu-)
 #                                          defaults to arm-linux-gnu- for arm
@@ -183,6 +185,9 @@
     ifeq ($(KERNEL_CC),)
         KERNEL_CC := CC="$(CCACHE_BIN) clang"
     endif
+    ifeq ($(KERNEL_LD),)
+        KERNEL_LD :=
+    endif
 endif
 
 ifneq ($(TARGET_KERNEL_MODULES),)
@@ -200,7 +205,7 @@
 # $(1): output path (The value passed to O=)
 # $(2): target to build (eg. defconfig, modules, dtbo.img)
 define internal-make-kernel-target
-$(PATH_OVERRIDE) $(KERNEL_MAKE_CMD) $(KERNEL_MAKE_FLAGS) -C $(KERNEL_SRC) O=$(KERNEL_BUILD_OUT_PREFIX)$(1) ARCH=$(KERNEL_ARCH) $(KERNEL_CROSS_COMPILE) $(KERNEL_CLANG_TRIPLE) $(KERNEL_CC) $(2)
+$(PATH_OVERRIDE) $(KERNEL_MAKE_CMD) $(KERNEL_MAKE_FLAGS) -C $(KERNEL_SRC) O=$(KERNEL_BUILD_OUT_PREFIX)$(1) ARCH=$(KERNEL_ARCH) $(KERNEL_CROSS_COMPILE) $(KERNEL_CLANG_TRIPLE) $(KERNEL_CC) $(KERNEL_LD) $(2)
 endef
 
 # Make a kernel target