BOOT_DEFAULTS: Tighten some dependencies

We only need some block filesystem related commands if BLK is enabled,
so set that. We only need USB things if we have USB_HOST, so also check
for that. Finally, we can change slightly the default for EFI_PARTITIONS
to be on BOOT_DEFAULTS rather than DISTRO_DEFAULTS.

Signed-off-by: Tom Rini <trini@konsulko.com>