binman: Provide general support for updating ELF symbols

The current support for updating variables in a binary is hard-coded to
work with U-Boot:

- It assumes the image starts at __image_copy_start
- It uses the existing U-Boot-specific entry types

It is useful for other projects to use these feature.

Add properties to enable writing symbols for any blob, a way of specifying
the base symbol and a way of providing the ELF filename to allow symbol
lookup to take place.

With this it is possible to update a Zephyr image, such as zephyr.bin
after it has been built.

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