f_sdp: Support searching and loading FIT or container image

Add support to f_sdp to search and load iMX8 container image or iMX8M
FIT image by new UUU command SDPV.

When using the SDPV, the uuu will continue to send out data after first
level boot loader used by ROM. This means uuu won't skip to the offset
of the second boot loader, and the padding data before second boot loader
will be sent out. So we have to search the FIT header or container header
in the buffer that SDP received.

Also change to more common method to exit f_sdp handler not depending on
SPL_FIT_FOUND flag because container loader won't set this.

Signed-off-by: Peng Fan <peng.fan@nxp.com>