[XS] Change in coreboot[master]: Makefile.inc: Use 'Wmissing-include-dirs' command option

[Tim Crawford (Code Review)]

Attention is currently required from: Arthur Heymans, Elyes Haouas, Yu-Ping Wu.

Tim Crawford has posted comments on this change. ( https://review.coreboot.org/c/coreboot/+/70251?usp=email )

Change subject: Makefile.inc: Use 'Wmissing-include-dirs' command option
......................................................................

Patch Set 34:

(1 comment)

Patchset:

PS34: 
This broke compiling with a custom FSP (using `FSP_HEADER_PATH`) if vboot is built.

Really, it's that vboot doesn't fix up include paths correctly (`vboot-fixup-includes`), so includes using absolute paths are broken.

e.g., what should be

-I"/home/tcrawford/Projects/System76/firmware-open/models/addw3/AlderLakeFspBinPkg/Include"

gets converted to

-I/home/tcrawford/Projects/System76/firmware-open/coreboot//home/tcrawford/Projects/System76/firmware-open/models/addw3/AlderLakeFspBinPkg/Include