Make:Allow for extension in tool names. ...

In some build environments executable programs have a specific file
extension.  The value of BIN_EXT is appended to the relevant tool file
names to allow for this.
The value of BIN_EXT is set, where appropriate, by the build environment
specific make helper (to .exe for Windows build environments).

.gitignore is updated to hide the new (.exe) files.

Change-Id: Icc32f64b750e425265075ad4e0dea18129640b86

Make:Use environment variables for OS detection. ...

Add make helper files to select the appropriate settings for the build
environment. Selection is made in make_helpers/build_env.mk, which
selects other files to include using generic build environment settings.
The Trusted Firmware Makefile and supporting tool Makefiles are updated
to include build_env.mk instead of unix.mk.

NOTE: This change does not fully enable builds in other build
      environments. It facilitates this without compromising the
      existing build environments.

Change-Id: Ic4064ffe6ce158bbd16d7cc9f27dd4655a3580f6