firmware: Change to use positive offset to access relocation entries

The codes currently skip the very first relocation entry, but later
reference the elements in the relocation entry using minus offsets.

Change to use positive offsets so that there is no need to skip the
first relocation entry.

Signed-off-by: Bin Meng <bmeng@tinylab.org>
Reviewed-by: Anup Patel <anup@brainfault.org>
Reviewed-by: Xiang W <wxjstz@126.com>