Give user's compiler flags precedence over default ones ...

The user can provide additional CFLAGS to use when building TF.
However, these custom CFLAGS are currently prepended to the
standard CFLAGS that are hardcoded in the TF build system. This
is an issue because when providing conflicting compiler flags
(e.g. different optimisations levels like -O1 and -O0), the last
one on the command line usually takes precedence. This means that
the user flags get overriden.

To address this problem, this patch separates the TF CFLAGS from
the user CFLAGS. The former are now stored in the TF_CFLAGS make
variable, whereas the CFLAGS make variable is untouched and reserved
for the user. The order of the 2 sets of flags is enforced when
invoking the compiler.

Fixes ARM-Software/tf-issues#350

Change-Id: Ib189f44555b885f1dffbec6015092f381600e560

Make:Improve version string generation portability ...

To get round problems encountered when building in a DOS build
environment the generation of the .o file containing build identifier
strings is modified.
The problems encounterred were:
   1. DOS echo doesn't strip ' characters from the output text.
   2. git is not available from CMD.EXE so the BUILD_STRING value needs
      some other origin.

A BUILD_STRING value of "development build" is used for now.

MAKE_BUILD_STRINGS is used to customise build string generation in a DOS
environment. This variable is not defined in the UNIX build environment
make file helper, and so the existing build string generation behaviour
is retained in these build environments.

NOTE: This commit completes a cumulative series aimed at improving
      build portability across development environments.
      This enables the build to run on several new build environments,
      if the relevant tools are available.
      At this point the build is tested on Windows 7 Enterprise SP1,
      using CMD.EXE, Cygwin and Msys (MinGW),as well as a native
      Linux envionment".  The Windows platform builds used
      aarch64-none-elf-gcc.exe 4.9.1.  CMD.EXE and Msys used Gnu
      Make 3.81, cygwin used Gnu Make 4.1.

CAVEAT: The cert_create tool build is not tested on the Windows
        platforms (openssl-for-windows has a GPL license).

Change-Id: Iaa4fc89dbe2a9ebae87e2600c9eef10a6af30251

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