Replace all SCP FW (BL0, BL3-0) references ...

This patch replaces all references to the SCP Firmware (BL0, BL30,
BL3-0, bl30) with the image terminology detailed in the TF wiki
(https://github.com/ARM-software/arm-trusted-firmware/wiki):

    BL0          -->  SCP_BL1
    BL30, BL3-0  -->  SCP_BL2
    bl30         -->  scp_bl2

This change affects code, documentation, build system, tools and
platform ports that load SCP firmware. ARM plaforms have been
updated to the new porting API.

IMPORTANT: build option to specify the SCP FW image has changed:

    BL30 --> SCP_BL2

IMPORTANT: This patch breaks compatibility for platforms that use BL2
to load SCP firmware. Affected platforms must be updated as follows:

    BL30_IMAGE_ID --> SCP_BL2_IMAGE_ID
    BL30_BASE --> SCP_BL2_BASE
    bl2_plat_get_bl30_meminfo() --> bl2_plat_get_scp_bl2_meminfo()
    bl2_plat_handle_bl30() --> bl2_plat_handle_scp_bl2()

Change-Id: I24c4c1a4f0e4b9f17c9e4929da815c4069549e58

TBB: add authentication framework documentation ...

This patch updates the user guide, adding instructions to build the
Trusted Firmware with Trusted Board Support using the new framework.

It also provides documentation about the framework itself, including
a detailed section about the TBBR implementation using the framework.

Change-Id: I0849fce9c5294cd4f52981e7a8423007ac348ec6

TBB: use SHA256 to generate the certificate signatures ...

This patch replaces SHA1 by SHA256 in the 'cert_create' tool, so
certificate signatures are generated according to the NSA Suite B
cryptographic algorithm requirements.

Documentation updated accordingly.

Change-Id: I7be79e6b2b62dac8dc78a4f4f5006e37686bccf6

TBB: Add documentation for Trusted Board Boot ...

This patch updates the user-guide.md with the various build options related to
Trusted Board Boot and steps to build a FIP image which includes this
support. It also adds a trusted-board-boot.md which describes the scope and
design of this feature.

Change-Id: Ifb421268ebf7e06a135684c8ebb04c94835ce061