Allow overriding the linker

This patch allows the user to override the value of the LD Makefile
variable. This feature can be used to force the use of the new Clang
linker.

Change-Id: I97ffeb18e48fa75346702a479d7dc1e8abcb3621
Signed-off-by: Roberto Vargas <roberto.vargas@arm.com>

Makefile

@@ -119,7 +119,7 @@ CC			:=	${CROSS_COMPILE}gcc
 CPP			:=	${CROSS_COMPILE}cpp
 AS			:=	${CROSS_COMPILE}gcc
 AR			:=	${CROSS_COMPILE}ar
-LD			:=	${CROSS_COMPILE}ld
+LINKER			:=	${CROSS_COMPILE}ld
 OC			:=	${CROSS_COMPILE}objcopy
 OD			:=	${CROSS_COMPILE}objdump
 NM			:=	${CROSS_COMPILE}nm
@@ -128,8 +128,8 @@ DTC			:=	dtc
 
 # Use ${LD}.bfd instead if it exists (as absolute path or together with $PATH).
 ifneq ($(strip $(wildcard ${LD}.bfd) \
-	$(foreach dir,$(subst :, ,${PATH}),$(wildcard ${dir}/${LD}.bfd))),)
-LD			:=	${LD}.bfd
+	$(foreach dir,$(subst :, ,${PATH}),$(wildcard ${dir}/${LINKER}.bfd))),)
+LINKER			:=	${LINKER}.bfd
 endif
 
 ifeq (${ARM_ARCH_MAJOR},7)
@@ -143,12 +143,15 @@ endif
 ifeq ($(notdir $(CC)),armclang)
 TF_CFLAGS_aarch32	=	-target arm-arm-none-eabi $(march32-directive)
 TF_CFLAGS_aarch64	=	-target aarch64-arm-none-eabi -march=armv8-a
+LD			=	$(LINKER)
 else ifneq ($(findstring clang,$(notdir $(CC))),)
 TF_CFLAGS_aarch32	=	$(target32-directive)
 TF_CFLAGS_aarch64	=	-target aarch64-elf
+LD			=	$(LINKER)
 else
 TF_CFLAGS_aarch32	=	$(march32-directive)
 TF_CFLAGS_aarch64	=	-march=armv8-a
+LD			=	$(LINKER)
 endif
 
 TF_CFLAGS_aarch32	+=	-mno-unaligned-access