binman: Provide a way to set the symbol base address

binman: Provide a way to set the symbol base address

The base address of the ELF containing symbols is normally added to
any symbols written, so that the value points to the correct address in
memory when everything is loaded. When the binary resides on disk, a
different offset may be needed, typically 0. Provide a way to specify
this.

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