create-tftf.c: add --sym flag that specifies a symbol name for the entry point
create-tftf.c: add --sym flag that specifies a symbol name for the entry point

Added a --sym/--start-sym flag that specifies a symbol name for the TFTF entry
point, to be used overriding the ELF entry point.  If --sym and --start are
passed together, it's an error.

Testing Done: produce a TFTF with its entry point set to --sym Reset_Handler
Bug Number: SW-2252
Review URL: https://reviews.leaflabs.com/r/2539/
Reviewed by: Morgan Girling <girling_morgan@projectara.com>, Jun Li <li_jun@projectara.com>

Signed-off-by: Eli Sennesh <5ebe47bba478951130dff4ea23416219c2dfa45d@leaflabs.com>

gregkh/bootrom-tools,projectara/bootrom-tools,projectara/bootrom-tools,gregkh/bootrom-tools,projectara/bootrom-tools,gregkh/bootrom-tools