The SCP_BL2 is transferred to SCP during BL2 image load and authenticate
sequence. The Boot-Over-MHU (BOM) protocol is used as transport for this. After
the SCP boots using the transferred image, the AP CPU waits till the `READY`
message is received from SCP. This patch separates the API for transport of
image from the wait for `READY` message and also moves the related files to
the `css/drivers` folder. The previous API `scp_bootloader_transfer` is
renamed to `css_scp_boot_image_xfer` to reflect the css naming convention.
This reorganisation also allows easier switch to a different transport
(eg: Shared Data Structure based transfer) in future

Change-Id: I8a96f9c4616ffde6dbfdf7c18f6f6f8bfa40bbf0
Signed-off-by: Soby Mathew <soby.mathew@arm.com>