Added new tool file to support legacy LiberoSoC 11.9 and ProASIC targets

Reasons for this addition:

There is an incompatibility between LiberoSoC v >= 12 and the legacy, 11.9 version.
V11.9 is the latest one that supports ProASIC3 devices.
The already supported LiberoSoC tool file was developed for v12, and for devices other than ProASIC3.
In order to not compromise the existing LiberoSoC v12 support (I have not yet worked with this version), I added a separate tool file for the legacy LiberoSoC v11.9.

In the top-level Manifest.py, the LiberoSoC11 can be specified as:

syn_tool = "liberosoc11"

NOTE: With this addition, I could only focus on ProASIC3(RT included) device support. Not tested with any other target devices families.

Added support for `Makefile.mk` includes to be included AFTER the generated targets and variable definitions in the `Makefile`

This way, the custom targets defined in the post-include Makefile.mk can utilize all the Makefile variables defined in the preamble of the HDLMake generated Makefile.
The initial reason for this addition is to add a bitstream file -> flash image conversion target for LiberoSoC (and potentially other tools).

In the top-level Manifest.py, a post-include .mk can be specified as:

incl_post_makefiles = ["proasic3_flash_conv.cygwin.mk"]