arm: add support for linking images entirely in SRAM

The image will be linked at a different address and with different
ROM/RAM sizes to allow running a bootloder image that loads a payload
image. The addresses/sizes depend on if it is a XIP image or not
(CONFIG_XIP), and in the case of a XIP image, if it is a bootloader
image (CONFIG_IS_BOOTLOADER) or not.

In the case of a bootloader, it is given the full ROM and
CONFIG_BOOTLOADER_SRAM_SIZE kB of SRAM.  When not a XIP image, it is given
the full SRAM minus CONFIG_BOOTLOADER_SRAM_SIZE kB, and is linked at the
start of SRAM.

Change-Id: Ibbb693c7bff022f313dac40f21c04a61f4bed115
Signed-off-by: Benjamin Walsh <benjamin.walsh@windriver.com>
Signed-off-by: Peter Mitsis <peter.mitsis@windriver.com>