Kbuild: Help text update.

This commit updates the help text content to cover
the targets that are currently supported.
The deleted help text will be added as functions are enabled.

Change-Id: Ia7c539a44427d740f23cdf12e5d3a18d3dd7cf24
Signed-off-by: Juan Manuel Cruz <juan.m.cruz.alcaraz@linux.intel.com>