Tools: set multiple variable with fw_setenv utility

Add a sort of batch mode to fw_setenv, allowing to set
multiple variables in one shot, without updating the flash after
each set as now. It is added the possibility to pass
a config file with a list of pairs <variable, value> to be set,
separated by a TAB character.

Signed-off-by: Stefano Babic <sbabic@denx.de>