Juno: Add support for image overlaying in Trusted SRAM

Juno: Add support for image overlaying in Trusted SRAM

This patch allows the BL3-1 NOBITS section to overlap the BL1 R/W
section since the former will always be used after the latter.
Similarly, the BL3-2 NOBITS section can overlay the BL2 image
when BL3-2 is loaded in Trusted SRAM.

Due to the current size of the images, there is no actual overlap.
Nevertheless, this reorganization may help to optimise the Trusted
SRAM usage when the images size grows.

Note that because BL3-1 NOBITS section is allowed to overlap the
BL1 R/W section, BL1 global variables will remain valid only until
execution reaches the BL3-1 entry point during a cold boot.

Documentation updated accordingly.

Fixes ARM-software/tf-issues#254

Change-Id: Id538f4d1c7f1f7858108280fd7b97e138572b879