binman: Convert FIT entry type to a subclass of Section entry type

The binman FIT entry type shares some code with the Section entry type.
This shared code is bound to grow, since FIT entries are conceptually a
variation of Section entries.

Make FIT entry type a subclass of Section entry type, simplifying it a
bit and providing us the features that Section implements. Also fix the
subentry alignment test which now attempts to write symbols to a
nonexistent SPL ELF test file by creating it first.

Signed-off-by: Alper Nebi Yasak <alpernebiyasak@gmail.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
Avoid AddMissingProperties() and SetCalculatedProperties() with FIT:
Signed-off-by: Simon Glass <sjg@chromium.org>