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>