[M] Change in coreboot[main]: tree: Use boolean for "eist_enable"

[Felix Singer (Code Review)]

Attention is currently required from: Alexander Couzens, Angel Pons, Christian Walter, Dinesh Gehlot, Elyes Haouas, Eran Mitrani, Erik van den Bogaert, Frans Hendriks, Jakub Czapiga, Jeremy Soller, Jonathon Hall, Kapil Porwal, Lean Sheng Tan, Michael Niewöhner, Michał Kopeć, Michał Żygowski, Nick Vaccaro, Patrick Rudolph, Piotr Król, Sean Rhodes, Subrata Banik, Tarun, Tim Crawford, Werner Zeh.

Felix Singer has posted comments on this change by Elyes Haouas. ( https://review.coreboot.org/c/coreboot/+/83575?usp=email )

Change subject: tree: Use boolean for "eist_enable"
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Patch Set 2:

(1 comment)

Patchset:

PS2: 
I would like to get in CB:77374 and others in first. Once that is done we can use a reproducible way for moving over the boolean options to true/false keywords.