binman: Add support for an x86 'reset' section

binman: Add support for an x86 'reset' section

At present binman has a single entry type for the 16-bit code code needed
to start up an x86 processor. This entry is intended to include both the
reset vector itself as well as the code to move to 32-bit mode.

However this is not very flexible since in some cases other data needs to
be included at the top of the SPI flash, in between these two pieces. For
example Intel requires that a FIT (Firmware Image Table) pointer be placed
0x40 bytes before the end of the ROM.

To deal with this, add a new reset entry for just the reset vector. A
subsequent change will adjust the existing 'start16' entry.

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