Refine Makefile on error handling for ARM32 architecture detection

With the original Makefile, the host without "arch" in PATH gets "make: arch:
No such file or directory" printed everytime while making, regardless which
target is choosed. The "2>/dev/null" is added for suppressing the error
message.

Author: idoleat

mk/common.mk

@@ -5,7 +5,7 @@ else
     PRINTF = env printf
 endif
 
-HOST_ARCH = $(shell arch)
+HOST_ARCH = $(shell arch 2>/dev/null)
 
 # Control the build verbosity
 ifeq ("$(VERBOSE)","1")