Hide error messages about nonexisting directories when scanning $PATH

$PATH can contains directories that do not exist, so hide error messages
about those entries.
Signed-off-by: Priit Laes <plaes@plaes.org>

Makefile

@@ -56,7 +56,7 @@ 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_COMPILE ?= $(or $(shell find $(PATH_DIRS) -executable -name 'arm*-gcc' -printf '%f\t' 2>/dev/null | cut -f 1 | sed -e 's/-gcc/-/'),$(CROSS_DEFAULT))
 CROSS_CC ?= $(CROSS_COMPILE)gcc
 
 DESTDIR ?=