Export - gcc arm makefile broken for windows

[0xc0170]

Latest master as of today, having problems with creating dirs:

To reproduce:

python tools\build.py -m K64F -t GCC_ARM -r
python tools\project.py -m K64F -i gcc_arm -n RTOS_1 -b

Run make in the exported project:

'[' is not recognized as an internal or external command,
operable program or batch file.
c:/Code/git_repo/github/mbed/.build/export/MBED_10/Makefile:70: warning: overriding commands for target `.s.o'
c:/Code/git_repo/github/mbed/.build/export/MBED_10/Makefile:67: warning: ignoring old commands for target `.s.o'
'[' is not recognized as an internal or external command,
operable program or batch file.
arm-none-eabi-gcc -mcpu=cortex-m0plus -mthumb  -c -x assembler-with-cpp -c -Wall -Wextra -Wno-unused-parameter -Wno-miss
ing-field-initializers -fmessage-length=0 -fno-exceptions -fno-builtin -ffunction-sections -fdata-sections -funsigned-ch
ar -MMD -fno-delete-null-pointer-checks -fomit-frame-pointer -mcpu=cortex-m0plus -mthumb -O2 -o hal\targets\cmsis\TARGET
_Freescale\TARGET_KLXX\TARGET_KL25Z\TOOLCHAIN_GCC_ARM\startup_MKL25Z4.o c:/Code/git_repo/github/mbed/.build/export/MBED_
10/hal\targets\cmsis\TARGET_Freescale\TARGET_KLXX\TARGET_KL25Z\TOOLCHAIN_GCC_ARM/startup_MKL25Z4.s
'[' is not recognized as an internal or external command,
operable program or batch file.
A subdirectory or file -p already exists.
Error occurred while processing: -p.
make[1]: *** [hal\common\assert.o] Error 1
make: *** [bin] Error 2

cc @theotherjimmy

[theotherjimmy]

I'll get a VM up and test.