boot: Use stdbool

boot: Use stdbool

The way the UEFI spec defines BOOLEAN is fully compatible to stdbool, so
it is perfectly safe to switch to it. Although any other values than 0/1
are undefined by the spec, we could theoretically have cases where a
sloppy firmware hands us a bad BOOLEAN (since gnu-efi/edk2 declare it
as uint8_t). So any uses where we pass a pointer to BOOLEAN are left
untouched.