config/arm: rename variables

config/arm: rename variables

Rename Arm build variables and values so that they better conform to Arm
specifications. Also rename generically sounding variable to names that
better capture what the variables hold.

Rename machine_args_generic to part_number_config_arm since the
variable contains more than just the generic machine args and is used
mainly as the fallback arm configuration.
Rename the default machine args to generic machine args to reflect that.
The rest of the variables are self-explanatory.

Signed-off-by: Juraj Linkeš <juraj.linkes@pantheon.tech>
Reviewed-by: Ruifeng Wang <ruifeng.wang@arm.com>
Reviewed-by: Honnappa Nagarahalli <honnappa.nagarahalli@arm.com>
Acked-by: Jerin Jacob <jerinj@marvell.com>
Tested-by: Jerin Jacob <jerinj@marvell.com>
Tested-by: Pavan Nikhilesh <pbhagavatula@marvell.com>
Tested-by: Vimal Chungath <vcchunga@amazon.com>
Tested-by: Dharmik Thakkar <dharmik.thakkar@arm.com>