Introduce convenience functions to build

This patch introduces two convenience functions to the build system:

  - assert_boolean: asserts that a given option is assigned either 0 or
    1 as values

  - add_define: helps add/append macro definitions to build tool command
    line. This also introduces the variable DEFINES which is used to
    collect and pass all relevant configurations to build tools

Change-Id: I3126894b034470d39858ebb3bd183bda681c7126
Reorganize build options

At present, various build options are initialized at various places in
the Makefile. This patch gathers all build option declarations at the
top of the Makefile and assigns them default values.

Change-Id: I9f527bc8843bf69c00cb754dc60377bdb407a951
Document summary of build options in user guide

Change-Id: I6bd077955bf3780168a874705974bbe72ea0f5f1
Add build configuration for timer save/restore

At present, non-secure timer register contents are saved and restored as
part of world switch by BL3-1. This effectively means that the
non-secure timer stops, and non-secure timer interrupts are prevented
from asserting until BL3-1 switches back, introducing latency for
non-secure services. Often, secure world might depend on alternate
sources for secure interrupts (secure timer or platform timer) instead
of non-secure timers, in which case this save and restore is
unnecessary.

This patch introduces a boolean build-time configuration NS_TIMER_SWITCH
to choose whether or not to save and restore non-secure timer registers
upon world switch. The default choice is made not to save and restore
them.

Fixes ARM-software/tf-issues#148

Change-Id: I1b9d623606acb9797c3e0b02fb5ec7c0a414f37e
Merge pull request #78 from jeenuv:tf-issues-148