Remove dashes from image names: 'BL3-x' --> 'BL3x' ...

This patch removes the dash character from the image name, to
follow the image terminology in the Trusted Firmware Wiki page:

    https://github.com/ARM-software/arm-trusted-firmware/wiki

Changes apply to output messages, comments and documentation.

non-ARM platform files have been left unmodified.

Change-Id: Ic2a99be4ed929d52afbeb27ac765ceffce46ed76

Move context management code to common location ...

The upcoming Firmware Update feature needs transitioning across
Secure/Normal worlds to complete the FWU process and hence requires
context management code to perform this task.

Currently context management code is part of BL31 stage only.
This patch moves the code from (include)/bl31 to (include)/common.
Some function declarations/definitions and macros have also moved
to different files to help code sharing.

Change-Id: I3858b08aecdb76d390765ab2b099f457873f7b0c

Reduce deep nesting of header files ...

Reduce the number of header files included from other header
files as much as possible without splitting the files. Use forward
declarations where possible. This allows removal of some unnecessary
"#ifndef __ASSEMBLY__" statements.

Also, review the .c and .S files for which header files really need
including and reorder the #include statements alphabetically.

Fixes ARM-software/tf-issues#31

Change-Id: Iec92fb976334c77453e010b60bcf56f3be72bd3e

Separate out BL2, BL3-1 and BL3-2 early exception vectors from BL1 ...

bl1/aarch64/early_exceptions.S used to be re-used by BL2, BL3-1 and
BL3-2.  There was some early SMC handling code in there that was not
required by the other bootloader stages.  Therefore this patch
introduces an even simpler exception vector source file for BL2,
BL3-1 and BL3-2.

Fixes ARM-software/tf-issues#38

Change-Id: I0244b80e9930b0f8035156a0bf91cc3e9a8f995d