Reorganize boot code.

Simplify keyboard handling in post_menu.c, and move to util.c.
Move remaining functions in post_menu.c to boot.c; remove post_menu.c.
Also, remove broken check for F12 when in boot menu.
Move BEV setup code from post.c to boot.c.
Move option rom BEV adding code from optionroms.c to boot.c.
Avoid calling BX_PANIC during boot if there is an alternative.
diff --git a/Makefile b/Makefile
index 186ef51..34a7535 100644
--- a/Makefile
+++ b/Makefile
@@ -12,7 +12,7 @@
         serial.c clock.c pic.c cdrom.c ps2port.c smpdetect.c resume.c \
         pnpbios.c pirtable.c
 SRC16=$(SRCBOTH) system.c disk.c apm.c pcibios.c vgahooks.c font.c
-SRC32=$(SRCBOTH) post.c shadow.c post_menu.c memmap.c coreboot.c boot.c \
+SRC32=$(SRCBOTH) post.c shadow.c memmap.c coreboot.c boot.c \
       acpi.c smm.c mptable.c smbios.c pciinit.c optionroms.c mtrr.c
 cc-option = $(shell if test -z "`$(1) $(2) -S -o /dev/null -xc \