Beautify "make help"

Changes to make the help text a bit more readable:
1) The "usage" part is now a one-liner
2) The supported platforms list is printed separately

Change-Id: I93e48a6cf1d28f0ef9f3db16ce17725e4dff33c9
Signed-off-by: John Tsichritzis <john.tsichritzis@arm.com>

Makefile

@@ -909,11 +909,13 @@ cscope:
 	${Q}cscope -b -q -k
 
 help:
-	@echo "usage: ${MAKE} PLAT=<${PLATFORM_LIST}> [OPTIONS] [TARGET]"
+	@echo "usage: ${MAKE} [PLAT=<platform>] [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 "platform = ${PLATFORM_LIST}"
+	@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 "