d/rules: Add platform target specifiers

Add support for multiple targets depending on the platform.
This is a preparation commit for supporting the Renesas R-Car platform
that uses bl31.srec and bl2.srec.

Also add a make_target variable that can be different than bl31 for some
platforms.

Signed-off-by: Detlev Casanova <detlev.casanova@collabora.com>

debian/rules

@@ -25,6 +25,9 @@ platforms_nodebug := sun50i_a64 imx8mq
 imx8mn_subplatforms := imx8mn imx8mn_uart4
 imx8mn_uart4_assigns := IMX_BOOT_UART_BASE=0x30a60000
 
+rk3328_targets := bl31/bl31.elf
+rk3399_targets := bl31/bl31.elf
+
 # Always set CROSS_COMPILE, which also works for native builds.
 # TARGET_BOARD only required for k3 platform, but safe to set as not used at all elsewhere.
 define build_platform
@@ -32,11 +35,14 @@ define build_platform
 	$(eval debug := $(2))
 	$(eval buildtype := $(3))
 	$(eval subplatforms := $(if $($(platform)_subplatforms), $($(platform)_subplatforms), $(platform)))
-	$(eval target := $(if $(filter rk3328 rk3399,$(platform)),bl31/bl31.elf,bl31.bin))
+	$(eval targets := $(if $($(platform)_targets), $($(platform)_targets), bl31.bin))
+	$(eval make_target := $(if $($(platform)_make_target), $($(platform)_make_target), bl31))
 	$(eval board := $(if $(filter k3,$(platform)),lite,))
 	$(foreach subplatform, $(subplatforms), \
-		CROSS_COMPILE=aarch64-linux-gnu- CFLAGS= LDFLAGS= dh_auto_build -- V=$(VERBOSE) DEBUG=$(debug) $($(subplatform)_assigns) PLAT=$(platform) TARGET_BOARD=$(board) bl31 ; \
-		install -m644 build/$(platform)/$(board)/$(buildtype)/$(target) -Dt build/renamed/$(subplatform) ; \
+		CROSS_COMPILE=aarch64-linux-gnu- CFLAGS= LDFLAGS= dh_auto_build -- V=$(VERBOSE) DEBUG=$(debug) $($(subplatform)_assigns) PLAT=$(platform) TARGET_BOARD=$(board) $(make_target) ; \
+		$(foreach target, $(targets), \
+			install -m644 build/$(platform)/$(board)/$(buildtype)/$(target) -Dt build/renamed/$(subplatform) ; \
+		) \
 		# For each subplatform, make is called using the same PLAT variable. If
 		# the build is not cleaned between each call, objects will remain the
 		# same, without rebuilding them.