boot: Provide our own EFI API headers

boot: Provide our own EFI API headers

We want to get away from gnu-efi and the only really usable source of
EFI headers would be EDK2, which is somewhat impractical to use and
quite large to require to be around just for some headers.

As a bonus point, the new headers are safe to be included in userspace
code.

This should not have any behavior changes as it is mostly changing
header includes. There are some renames to conform to standard names
and a few minor device path fixups as the struct is defined slightly
different.

Of note is that this removes usage of uchar.h and wchar.h as they are
not guaranteed to be available in a freestanding environment. Instead
efi.h will provide the needed types.