Update help message printed by the Makefile
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

athoelke/arm-trusted-firmware,idl3r/arm-trusted-firmware,davwan01/arm-trusted-firmware,robixnai/arm-trusted-firmware,lsigithub/arm-trusted-firmware_public,sandrine-bailleux/arm-trusted-firmware,idl3r/arm-trusted-firmware,athoelke/arm-trusted-firmware,lsigithub/arm-trusted-firmware_public,sandrine-bailleux-arm/arm-trusted-firmware,sandrine-bailleux/arm-trusted-firmware,davwan01/arm-trusted-firmware,jcastillo-arm/arm-trusted-firmware,sbranden/arm-trusted-firmware,sbranden/arm-trusted-firmware,robixnai/arm-trusted-firmware,jcastillo-arm/arm-trusted-firmware,achingupta/arm-trusted-firmware,sandrine-bailleux-arm/arm-trusted-firmware,achingupta/arm-trusted-firmware,mtk09422/arm-trusted-firmware,mtk09422/arm-trusted-firmware