arm: afs: Set the filesize env. var. on load

The `afs load` command copies data to RAM. Set the filesize
environment variable to record how much data was 'loaded'

Signed-off-by: Robert Catherall <robert.catherall@arm.com>
Reviewed-by: Andre Przywara <andre.przywara@arm.com>
Tested-by: Andre Przywara <andre.przywara@arm.com>