fix(arm_fgpa): allow build after MAKE_* changes

Commit 434d0491c5 ("refactor(makefile): remove BL prefixes in build
macros") changed the MAKE_S macro to expect "bl31" instead of just "31".

Adjust our calls to MAKE_S and MAKE_LD to fix the build for arm_fpga.

Change-Id: I2743e421c10eaecb39bfa4515ea049a1b8d18fcb
Signed-off-by: Andre Przywara <andre.przywara@arm.com>

plat/arm/board/arm_fpga/platform.mk

@@ -117,9 +117,9 @@ BL31_SOURCES		+=	common/fdt_wrappers.c				\
 				${FPGA_CPU_LIBS}				\
 				${FPGA_GIC_SOURCES}
 
-$(eval $(call MAKE_S,$(BUILD_PLAT),plat/arm/board/arm_fpga/rom_trampoline.S,31))
-$(eval $(call MAKE_S,$(BUILD_PLAT),plat/arm/board/arm_fpga/kernel_trampoline.S,31))
-$(eval $(call MAKE_LD,$(BUILD_PLAT)/build_axf.ld,plat/arm/board/arm_fpga/build_axf.ld.S,31))
+$(eval $(call MAKE_S,$(BUILD_PLAT),plat/arm/board/arm_fpga/rom_trampoline.S,bl31))
+$(eval $(call MAKE_S,$(BUILD_PLAT),plat/arm/board/arm_fpga/kernel_trampoline.S,bl31))
+$(eval $(call MAKE_LD,$(BUILD_PLAT)/build_axf.ld,plat/arm/board/arm_fpga/build_axf.ld.S,bl31))
 
 bl31.axf: bl31 dtbs ${BUILD_PLAT}/rom_trampoline.o ${BUILD_PLAT}/kernel_trampoline.o ${BUILD_PLAT}/build_axf.ld
 	$(ECHO) "  LD      $@"