binman: Register and check bintools from FIT subentries

Binman keeps track of binary tools each entry wants to use. The
implementation of this for the FIT entry only adds "mkimage", but not
the tools that would be used by its subentries.

Register the binary tools that FIT subentries will use in addition to
the one FIT itself uses, and check their existence by copying the
appropriate method from Section entry type. Also add tests that check if
these subentries can use and warn about binary tools.

Signed-off-by: Alper Nebi Yasak <alpernebiyasak@gmail.com>
Reviewed-by: Simon Glass <sjg@chromium.org>