conf: simplify and comment main target.config file
conf: simplify and comment main target.config file

The main target configuration file has been simplified and some comments
added to better describe the different configuration options.

Signed-off-by: Patrick Bellasi <431c86f368affbc3148c4669fbd465131f7e82cb@arm.com>

joelagnel/lisa,bjackman/lisa,arnoldlu/lisa,credp/lisa,mdigiorgio/lisa,ARM-software/lisa,joelagnel/lisa,credp/lisa,ARM-software/lisa,credp/lisa,arnoldlu/lisa,bjackman/lisa,ARM-software/lisa,ARM-software/lisa,credp/lisa,JaviMerino/lisa,mdigiorgio/lisa