Boot1 tries to clear boot2's BSS. It makes several assumptions that are

Boot1 tries to clear boot2's BSS.  It makes several assumptions that are
incorrect (or incorrect now), and doesn't really know how much BSS to clear
anyway.  Remove the BSS clearing code from boot1 and add BSS clearing code
to the boot2 (in particular, /usr/src/sys/boot/i386/btx/lib/btxcsu.S).

This should solve numerous issues including boot2 getting confused about
the default boot device file path.

Collapse some boot2 code to make it fit again.

Move the BOOT2 virtual origin (relative to the BTX client address space)
into bootasm.h, replace an undocumented hardwired BOOT2 origin calculation
(that was SIZ_PAG*2) with BOOT2_VORIGIN.   Replace the hardwired ORG2
constant in boot2's Makefile with code to pull BOOT2_VORIGIN out of the
header file.