binman: Support new op-tee binary format

OP-TEE has a format with a binary header that can be used instead of the
ELF file. With newer versions of OP-TEE this may be required on some
platforms.

Add support for this in binman. First, add a method to obtain the ELF
sections from an entry, then use that in the FIT support. We then end up
with the ability to support both types of OP-TEE files, depending on which
one is passed in with the entry argument (TEE=xxx in the U-Boot build).

Signed-off-by: Simon Glass <sjg@chromium.org>