scripts-and-docs/u-boot: add note for mmc renumbering
scripts-and-docs/u-boot: add note for mmc renumbering

later this will go into u-boot defaults

Signed-off-by: Andreas Müller <d72a15aaaca3a7afa855175c72dc8d2e05931e32@googlemail.com>

schnitzeltony/meta-variscite-community,schnitzeltony/meta-variscite-community