kleaf: Add bzlmod files.

... for bzlmod builds.

Also move extra toolchains to a command line option
because toolchains in WORKSPACE.bzlmod are registered
after those in MODULE.bazel. We need to register
//u-boot:py_toolchain with the highest priority.

Bug: 276493276
Test: TH
Change-Id: If64f88c3a33e8a44012ca102bad27eafba21d41d
3 files changed