libs/utils/env: add support for custom board specified via JSON file
libs/utils/env: add support for custom board specified via JSON file

This allows to specify a custom board configuration using a JSON file located
under libs/utils/platforms.
A platform can be specified by reporting these properties:
    "cores"    : list of cores identifiers
                 unique names are used to defined clusters
    "big_core" : the name used to represent big CPUs in a big.LITTLE target
    "modules"  : (optional) list of modules to load for that target

Signed-off-by: Patrick Bellasi <431c86f368affbc3148c4669fbd465131f7e82cb@arm.com>
Signed-off-by: Michele Di Giorgio <99fd0b6f5a4e02a4022988691284da2a0299b046@arm.com>

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