Fix the documentation and DOS port

Update the DOS cross-compile documentation,
and workaround issue with valloc() with the
latest DJGPP.

Change-Id: I909c5635aec5076440d2fde73d943f8ad10b8051
Signed-off-by: Rudolf Marek <r.marek@assembler.cz>
Reviewed-on: https://review.coreboot.org/23044
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Nico Huber <nico.h@gmx.de>
---
M README
M physmap.c
2 files changed, 53 insertions(+), 26 deletions(-)