Improve BL3-0 documentation

Provide some information about the expected use of BL3-0.

Fixes ARM-software/tf-issues#144

Change-Id: I5c8d59a675578394be89481ae4ec39ca37522750

docs/firmware-design.md

@@ -219,6 +219,15 @@ access to access controlled components. On the Base FVP a TrustZone controller
 abstraction layer is initialized which is used to load further bootloader
 images.
 
+#### BL3-0 (System Control Processor Firmware) image load
+
+Some systems have a separate System Control Processor (SCP) for power, clock,
+reset and system control. BL2 loads the optional BL3-0 image from platform
+storage into a platform-specific region of secure memory. The subsequent
+handling of BL3-0 is platform specific. Typically the image is transferred into
+SCP memory using a platform-specific protocol. The SCP executes BL3-0 and
+signals to the Application Processor (AP) for BL2 execution to continue.
+
 #### BL3-1 (EL3 Runtime Firmware) image load
 
 BL2 loads the BL3-1 image from platform storage into a platform-specific address

tools/fip_create/fip_create.c

@@ -53,7 +53,7 @@ uuid_t uuid_null = {0};
  * const char* format_type_str[] = { "RAW", "ELF", "PIC" };
  */
 
-/* Currently only BL2 and BL31 images are supported. */
+/* The images used depends on the platform. */
 static entry_lookup_list_t toc_entry_lookup_list[] = {
 	{ "Trusted Boot Firmware BL2", UUID_TRUSTED_BOOT_FIRMWARE_BL2,
 	  "bl2", NULL, FLAG_FILENAME },