platforms: add support for boot_info.sh

Build scripts now save a file called boot_info.sh in the output
directory. Source this file in boot scripts to set the necessary
flags/variables required to setup the platform for booting.

Signed-off-by: Rakshit Goyal <rakshit.goyal@arm.com>
Change-Id: I27e256b91214d227ce7a8abbf38d3b8ffe69b182