Detect toolchain automatically

Detect toolchain automatically by creating toolchain list,
and check the items in list is installed or not.

If there are multiple toolchain installed, get the first
item in valid toolchain list.

Author: Risheng1128

mk/toolchain.mk

@@ -9,24 +9,14 @@ else
 endif
 
 # Validate GNU Toolchain for RISC-V
-CROSS_COMPILE ?= riscv32-unknown-elf-
-RV32_CC = $(CROSS_COMPILE)gcc
-RV32_CC := $(shell which $(RV32_CC))
-ifndef RV32_CC
-    # xPack GNU RISC-V Embedded GCC
-    CROSS_COMPILE = riscv-none-elf-
-    RV32_CC = $(CROSS_COMPILE)gcc
-    RV32_CC := $(shell which $(RV32_CC))
-    ifndef RV32_CC
-        # DEPRECATED: Replaced by xpack-dev-tools/riscv-none-elf-gcc-xpack
-        CROSS_COMPILE = riscv-none-embed-
-        RV32_CC = $(CROSS_COMPILE)gcc
-        RV32_CC := $(shell which $(RV32_CC))
-        ifndef RV32_CC
-        $(warning No GNU Toolchain for RISC-V found.)
-        CROSS_COMPILE :=
-        endif
-    endif
-endif
+TOOLCHAIN_LIST := riscv-none-elf-      \
+		  riscv32-unknown-elf- \
+		  riscv64-unknown-elf-
+
+VALID_TOOLCHAIN := $(foreach toolchain,$(TOOLCHAIN_LIST),     \
+		   $(shell which $(toolchain)gcc > /dev/null) \
+		   $(if $(filter 0,$(.SHELLSTATUS)),$(toolchain)))
 
+# Get the first element in valid toolchain list
+CROSS_COMPILE := $(word 1,$(VALID_TOOLCHAIN))
 export CROSS_COMPILE