[build] Add named configuration for VirtualBox

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

src/config/vbox/README

@@ -0,0 +1,16 @@
+Build using this command line:
+
+make CONFIG=vbox EMBED=config/vbox/embedded.ipxe bin/intel--virtio-net--pcnet32.rom
+
+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

src/config/vbox/embedded.ipxe

@@ -0,0 +1,5 @@
+#!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

@@ -0,0 +1,28 @@
+/* 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
+#undef SANBOOT_PROTO_FCP
+#undef REBOOT_CMD
+#undef CPUID_CMD
+
+/* Disabled from config/general.h */
+
+#undef DOWNLOAD_PROTO_HTTP
+#undef CRYPTO_80211_WEP
+#undef CRYPTO_80211_WPA
+#undef CRYPTO_80211_WPA2
+#undef IWMGMT_CMD
+#undef FCMGMT_CMD
+#undef SANBOOT_CMD
+#undef MENU_CMD
+#undef LOGIN_CMD
+#undef SYNC_CMD
+
+/* Ensure ROM banner is not displayed */
+
+#undef ROM_BANNER_TIMEOUT
+#define ROM_BANNER_TIMEOUT 0