infra/common: document busybox boot procedure

Add a new document that explains the procedure to boot busybox
filesystem distributions on the Neoverse Reference Design platforms.

Change-Id: I8eba5e5f0d1774d2f4938caef6d31740fb0c0b4e
Signed-off-by: Aditya Angadi <aditya.angadi@arm.com>
Signed-off-by: Vijayenthiran Subramaniam <vijayenthiran.subramaniam@arm.com>