bootstd: Create a new BOOTMETH_DISTRO

We cannot be sure what bootmeth a distro will need to use. Add a new
BOOTMETH_DISTRO option which collects these together. Select this from
BOOTSTD_DEFAULTS so that it is clear what is needed.

Signed-off-by: Simon Glass <sjg@chromium.org>
[trini: Make BOOTMETH_EFILOADER depend on EFI_LOADER, select if EFI_LOADER]
Signed-off-by: Tom Rini <trini@konsulko.com>