[build] Avoid using embedded script in VirtualBox named configuration

Signed-off-by: Michael Brown <mcb30@ipxe.org>

src/config/vbox/README

 Build using this command line:
 
-make CONFIG=vbox EMBED=config/vbox/embedded.ipxe bin/intel--virtio-net--pcnet32.rom
+make CONFIG=vbox bin/intel--virtio-net--pcnet32.isarom
 
 Max size of a VirtualBox ROM is 56KB, 57344 bytes. There should be no need
 to pad the image as long as the binary is smaller or equal to this size.
 
-The embedded script is required because VirtualBox uses the ROM as an ISA
-ROM, which will not perform any autoboot behavior.  The bundled embedded
-script reproduces the default autoboot behavior.
-
 To use the ROM in VirtualBox you need to enable it using this command:
 
 vboxmanage setextradata global \
     VBoxInternal/Devices/pcbios/0/Config/LanBootRom \
-    path/to/intel--virtio-net--pcnet32.rom
+    /absolute/path/to/intel--virtio-net--pcnet32.isarom
+
+NB: If you build the ROM using the .rom prefix then it'll be built as a PCI
+ROM, which won't work properly in VirtualBox.  The error message you'll see
+is "No more network devices", which is somewhat confusing.  If you enter the
+shell and use the "autoboot" command things will work as intended.  Remember
+to always build as a .isarom to avoid this issue.

src/config/vbox/embedded.ipxe

-#!ipxe
-prompt --key 0x02 --timeout 2000 Press Ctrl-B to enter the iPXE shell... && shell || goto auto
-exit
-:auto
-autoboot

src/config/vbox/general.h

 /* Disabled from config/defaults/pcbios.h */
 
 #undef IMAGE_ELF
-#undef IMAGE_MULTIBOOT
 #undef SANBOOT_PROTO_ISCSI
 #undef SANBOOT_PROTO_AOE
 #undef SANBOOT_PROTO_IB_SRP