Add descriptor based image management support in BL1

Add descriptor based image management support in BL1

As of now BL1 loads and execute BL2 based on hard coded information
provided in BL1. But due to addition of support for upcoming Firmware
Update feature, BL1 now require more flexible approach to load and
run different images using information provided by the platform.

This patch adds new mechanism to load and execute images based on
platform provided image id's. BL1 now queries the platform to fetch
the image id of the next image to be loaded and executed. In order
to achieve this, a new struct image_desc_t was added which holds the
information about images, such as: ep_info and image_info.

This patch introduces following platform porting functions:

unsigned int bl1_plat_get_next_image_id(void);
	This is used to identify the next image to be loaded
	and executed by BL1.

struct image_desc *bl1_plat_get_image_desc(unsigned int image_id);
	This is used to retrieve the image_desc for given image_id.

void bl1_plat_set_ep_info(unsigned int image_id,
struct entry_point_info *ep_info);
	This function allows platforms to update ep_info for given
	image_id.

The plat_bl1_common.c file provides default weak implementations of
all above functions, the `bl1_plat_get_image_desc()` always return
BL2 image descriptor, the `bl1_plat_get_next_image_id()` always return
BL2 image ID and `bl1_plat_set_ep_info()` is empty and just returns.
These functions gets compiled into all BL1 platforms by default.

Platform setup in BL1, using `bl1_platform_setup()`, is now done
_after_ the initialization of authentication module. This change
provides the opportunity to use authentication while doing the
platform setup in BL1.

In order to store secure/non-secure context, BL31 uses percpu_data[]
to store context pointer for each core. In case of BL1 only the
primary CPU will be active hence percpu_data[] is not required to
store the context pointer.

This patch introduce bl1_cpu_context[] and bl1_cpu_context_ptr[] to
store the context and context pointers respectively. It also also
re-defines cm_get_context() and cm_set_context() for BL1 in
bl1/bl1_context_mgmt.c.

BL1 now follows the BL31 pattern of using SP_EL0 for the C runtime
environment, to support resuming execution from a previously saved
context.

NOTE: THE `bl1_plat_set_bl2_ep_info()` PLATFORM PORTING FUNCTION IS
      NO LONGER CALLED BY BL1 COMMON CODE. PLATFORMS THAT OVERRIDE
      THIS FUNCTION MAY NEED TO IMPLEMENT `bl1_plat_set_ep_info()`
      INSTEAD TO MAINTAIN EXISTING BEHAVIOUR.

Change-Id: Ieee4c124b951c2e9bc1c1013fa2073221195d881