loader: create bio_alloc and bio_free for bios bounce buffer

We do have 16KB buffer space defined in pxe.c, move it to bio.c and implement
bio_alloc()/bio_free() interface to make it possible to use this space for
other BIOS calls (notably, from biosdisk.c).

MFC after: 2 weeks
Differential Revision: https://reviews.freebsd.org/D17131