[L] Change in coreboot[master]: mainboard: Add SPDX license headers to Makefiles

[9elements QA (Code Review)]

9elements QA has posted comments on this change. ( https://review.coreboot.org/c/coreboot/+/68982?usp=email )

Change subject: mainboard: Add SPDX license headers to Makefiles
......................................................................

Patch Set 4:

Automatic boot test returned (PASS/FAIL/TOTAL): 6 / 1 / 7

PASS: x86_32  "QEMU x86 q35/ich9" , build config EMULATION_QEMU_X86_Q35_SMM_TSEG 
     and payload SeaBIOS : https://lava.9esec.io/r/177224
FAIL: x86_32  "QEMU x86 q35/ich9" , build config EMULATION_QEMU_X86_Q35 
     and payload TianoCore : https://lava.9esec.io/r/177223
PASS: x86_32  "QEMU x86 q35/ich9" , build config EMULATION_QEMU_X86_Q35 
     and payload SeaBIOS : https://lava.9esec.io/r/177222
PASS: x86_64  "QEMU x86 i440fx/piix4" , build config EMULATION_QEMU_X86_I440FX_X86_64 
     and payload SeaBIOS : https://lava.9esec.io/r/177221
PASS: x86_32  "QEMU x86 i440fx/piix4" , build config EMULATION_QEMU_X86_I440FX_ASAN 
     and payload SeaBIOS : https://lava.9esec.io/r/177220
PASS: x86_32  "QEMU x86 i440fx/piix4" , build config EMULATION_QEMU_X86_I440FX_ 
     and payload SeaBIOS : https://lava.9esec.io/r/177219
PASS: x86_32  "QEMU x86 i440fx/piix4" , build config EMULATION_QEMU_X86_I440FX 
     and payload SeaBIOS : https://lava.9esec.io/r/177218

Please note: This test is under development and might not be accurate at all!