Fix a memcmp size issue
Fix a memcmp size issue

The original code compares sentinel value of config data to
sizeof(secondstage_cfg_sentinel), but that actually included the
trailing '\0'. And it did work because the config data contained
0 after the sentinel value in coincident.

Testing Done:   Config data still works
Bug Number:     Trivial
Review URL:     https://reviews.leaflabs.com/r/2225/
Reviewed by:    Morgan Girling, Eli Sennesh

Signed-off-by:  Jun Li <4a139eac5426aee89571cee0a0f52cc73d82e960@projectara.com>

projectara/bootrom,gregkh/bootrom