[SRCREV] set correct U-boot SRCREV for RK3568 and RK3588

Different boards use versions of u-boot that might reside in different branches of u-boot-seco-rk. The SRCREV.conf file specifies the hash of the commit to use in the main branch and it gets constantly updated by the CI/CD scripts when a new revision is committed to the branch.

This commit introduces a new convention for handling different branches of U-boot. Each recipe sets an override, which is the name of the branch sanitized for use as an override and prefixed with the string "u-boot-". When a branch is updated, the CI/CD system will look in SRCREV.conf for an override of a specific branch (defaulting to the master branch).