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.

mk/toolchain.mk

@@ -9,24 +9,20 @@ 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
+TOOLCHAIN_LIST := riscv-none-elf-      \
+		  riscv32-unknown-elf- \
+		  riscv64-unknown-elf- \
+		  riscv-none-embed-
+
+# TODO: add support to clang/llvm based cross compilers
+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))
+ifeq ($(CROSS_COMPILE),)
+$(warning GNU Toolchain for RISC-V is required. Please check package installation)
 endif
 
 export CROSS_COMPILE