Have t430-dgpu boards apply Dual Graphics at build time

[tlaurion]

@weyounsix Please take a look at latest commit

This modifies Makefile and modules/coreboot so that nvramtool is build against host tools.
Then, nvramtool is used in both t430-dgpu boards to have Dual Graphics added into dgpu ROM, which is then splitted into top and bottom rom.

Build happening under https://app.circleci.com/pipelines/github/tlaurion/heads/1208/workflows/70b2fd69-8cd5-423a-a24c-375d1f4868be

Locally tested:

user@heads-tests:~/heads$ ~/heads/build/x86/coreboot-4.13/util/nvramtool/nvramtool -C build/x86/t430-dgpu-hotp-maximized/heads-t430-dgpu-hotp-maximized-v0.2.1.bis2-149-g626731e6-dirty-dual_gpu.rom -a
boot_option = Fallback
reboot_counter = 0x0
debug_level = Debug
nmi = Enable
power_on_after_fail = Disable
first_battery = Primary
bluetooth = Enable
wwan = Enable
touchpad = Enable
wlan = Enable
trackpoint = Enable
fn_ctrl_swap = Disable
sticky_fn = Disable
usb_always_on = Disable
sata_mode = AHCI
backlight = Both
gfx_uma_size = 32M
hybrid_graphics_mode = Dual Graphics
volume = 0x3