BaseTools: Rationalise makefile generation

BaseTools: Rationalise makefile generation

The GenMake.py script tests the platform environment
to determine the type of makefile that needs to be
generated. If a Windows build host is detected, the
makefile generated is of Nmake type. Otherwise a
GNUmake type is generated.

Furthermore, the <TARGET>_<TAGNAME>_<ARCH>_MAKE_PATH
option in tools_def.template defines the make tool
to use.
E.g.: for VS2017 this is configured to use Nmake, cf.
*_VS2017_*_MAKE_PATH = DEF(VS2017_BIN_HOST)\nmake.exe
while for GCC5 it is setup to use GNU make.
*_GCC5_*_MAKE_PATH = DEF(GCC_HOST_PREFIX)make

This prevents using the GCC compiler toolchain on a
Windows build host.

To address this issue this patch introduces 2 factors
to determine the generated makefile output.
  1. Platform -> to determine shell commands used
                 in makefile.
  2. MakeTool -> to determine the type of makefile
                 that needs to be generated.

Signed-off-by: Pierre Gondois <pierre.gondois@arm.com>
Signed-off-by: Sami Mujawar <sami.mujawar@arm.com>
Reviewed-by: Bob Feng <bob.c.feng@intel.com>