stand: Allocate bootinfo rather than have it be static

This saves 80 bytes (the new bootinfo structure was 84 bytes, and a
pointer is 4 bytes). The bi_load32 code is the same size.

Sponsored by: Netflix
Reviewed by: tsoome
Differential Revision: https://reviews.freebsd.org/D36575

(cherry picked from commit 9758dd3de1cddc8271be8dd6fee69286c5c86535)

stand: Pass in the proper size for bootinfo

Missed one sizeof(bi) -> sizeof(*bi) in 9758dd3de1cdd conversion to
allocating bootinfo.

Noticed by: tijl@
Fixes: 9758dd3de1cdd
Sponsored by: Netflix

(cherry picked from commit 4a676571e382d0188b98134732f421df57a47a87)