binman: Add a utility library for coreboot CBFS

binman: Add a utility library for coreboot CBFS

Coreboot uses a simple flash-based filesystem called Coreboot Filesystem
(CBFS) to organise files used during boot. This allows files to be named
and their position in the flash to be set. It has special features for
dealing with x86 devices which typically memory-map their SPI flash to the
top of 32-bit address space and need a 'boot block' ending there.

Create a library to help create and read CBFS files. This includes a
writer class, a reader class and associated other helpers. Only a subset
of features are currently supported.

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