Update help message printed by the Makefile

The message printed by 'make help' is incomplete. It doesn't
mention all relevant supported targets. This patch updates it.

The format of the first line of the help message has been changed
so that it no longer lists all supported targets. This eases the
maintenance as we don't need to update the list in 2 places
anymore whenever a new target is added.

Also add a reference to the user guide to get the list of
supported options.

Change-Id: I79d8b815b0ffc0c43b4c05124378fce0e938365c
diff --git a/Makefile b/Makefile
index 1e2f324..ae68dc7 100644
--- a/Makefile
+++ b/Makefile
@@ -625,16 +625,23 @@
 	${Q}cscope -b -q -k
 
 help:
-	@echo "usage: ${MAKE} PLAT=<${HELP_PLATFORMS}> <all|bl1|bl2|bl31|distclean|clean|checkcodebase|checkpatch>"
+	@echo "usage: ${MAKE} PLAT=<${HELP_PLATFORMS}> [OPTIONS] [TARGET]"
 	@echo ""
 	@echo "PLAT is used to specify which platform you wish to build."
 	@echo "If no platform is specified, PLAT defaults to: ${DEFAULT_PLAT}"
 	@echo ""
+	@echo "Please refer to the User Guide for a list of all supported options."
+	@echo "Note that the build system doesn't track dependencies for build "
+	@echo "options. Therefore, if any of the build options are changed "
+	@echo "from a previous build, a clean build must be performed."
+	@echo ""
 	@echo "Supported Targets:"
-	@echo "  all            Build the BL1, BL2 and BL31 binaries"
+	@echo "  all            Build all individual bootloader binaries"
 	@echo "  bl1            Build the BL1 binary"
 	@echo "  bl2            Build the BL2 binary"
-	@echo "  bl31           Build the BL31 binary"
+	@echo "  bl31           Build the BL3-1 binary"
+	@echo "  bl32           Build the BL3-2 binary"
+	@echo "  fip            Build the Firmware Image Package (FIP)"
 	@echo "  checkcodebase  Check the coding style of the entire source tree"
 	@echo "  checkpatch     Check the coding style on changes in the current"
 	@echo "                 branch against BASE_COMMIT (default origin/master)"
@@ -644,7 +651,7 @@
 	@echo "  certtool       Build the Certificate generation tool"
 	@echo "  fiptool        Build the Firmware Image Package(FIP) creation tool"
 	@echo ""
-	@echo "note: most build targets require PLAT to be set to a specific platform."
+	@echo "Note: most build targets require PLAT to be set to a specific platform."
 	@echo ""
 	@echo "example: build all targets for the FVP platform:"
 	@echo "  CROSS_COMPILE=aarch64-none-elf- make PLAT=fvp all"