arm: clang: Update support slightly

- Move most of the flags required into LLVM_RELFLAGS to test at build
  time instead of requiring them to be passed in.
- Update doc/README.clang to reflect this
- Switch to rpi_2 as the example as it's closer to working out of the
  box than rpi is.

Cc: Jeroen Hofstee <jeroen@myspectrum.nl>
Signed-off-by: Tom Rini <trini@konsulko.com>