release.7: Update RPI example + CHROOTDIR + SPDX

+ arm/RPI-B.conf no longer exists, adjust for arm64/RPI.conf
+ document default CHROOTDIR, also add to FILES list
+ tag SPDX

MFC after: 3 days
Reviewed by: cperciva
Differential Revision: https://reviews.freebsd.org/D53078

(cherry picked from commit 8a9f1a2286689c447e8528dc79ee6c4a47423fdc)