Correct batch detection algorimthm

There are two shell-like things that we care about
 bash: does environment varible expansion with $
 batch: does environment varible expansion by surrounding something with %

so to detect batch, try to expand a variable on the shell with $.
If you get a $ back, you are in batch!

tools/export/gcc_arm_common.tmpl

@@ -2,7 +2,7 @@
 # see http://mbed.org/handbook/Exporting-to-GCC-ARM-Embedded
 
 # cross-platform directory manipulation
-ifeq ($(OSTYPE),)
+ifeq ($(shell echo $$OS),$$OS)
     MAKEDIR = if not exist "$(1)" mkdir "$(1)"
     RM = rmdir /S /Q
 else