Makefile: Improve auto-detection of ARM cross compiler

Try an 'educated guess' for a suitable toolchain if no explicit
CROSS_COMPILE was set (but still default to "arm-none-eabi-").
Signed-off-by: Bernhard Nortmann <bernhard.nortmann@web.de>

Makefile

@@ -52,9 +52,12 @@ MISC_TOOLS = phoenix_info sunxi-nand-image-builder
 # Note: To use this target, set/adjust CROSS_COMPILE and MKSUNXIBOOT if needed
 BINFILES = jtag-loop.sunxi fel-sdboot.sunxi uart0-helloworld-sdboot.sunxi
 
-CROSS_COMPILE ?= arm-none-eabi-
-CROSS_CC ?= $(CROSS_COMPILE)gcc
 MKSUNXIBOOT ?= mksunxiboot
+PATH_DIRS := $(shell echo $$PATH | sed -e 's/:/ /g')
+# Try to guess a suitable default ARM cross toolchain
+CROSS_DEFAULT := arm-none-eabi-
+CROSS_COMPILE ?= $(or $(shell find $(PATH_DIRS) -executable -name 'arm*-gcc' -printf '%f\t' | cut -f 1 | sed -e 's/-gcc/-/'),$(CROSS_DEFAULT))
+CROSS_CC ?= $(CROSS_COMPILE)gcc
 
 DESTDIR ?=
 PREFIX  ?= /usr/local

README.md

@@ -112,8 +112,9 @@ and our default target (when simply using `make`).
 
 * `make target-tools`
 builds tools that are intended for the target (Allwinner SoC), using a
-cross-compiler. The toolchain prefix *CROSS_COMPILE* defaults to `arm-none-eabi-`,
-adjust it if needed.
+cross-compiler. The Makefile will try to auto-detect a suitable toolchain
+prefix, and falls back to `arm-none-eabi-` otherwise.
+If needed, you may override this by explicitly setting *CROSS_COMPILE*.
 <br>_Hint:_ When compiling 'natively' on the target platform you may
 simply use an empty toolchain prefix here (`make target-tools CROSS_COMPILE=`
 or `make all CROSS_COMPILE=`).