Allow efi_loader header to be included always

Allow efi_loader header to be included always

It is bad practice to put function declarations behind an #ifdef since
it makes it impossible to use IS_ENABLED() in the C code. The main reason
for doing this is when an empty static inline function is desired when
the feature is disabled.

To this end, this header provides two different versions of various
functions and macros. Collect them together in one place for clarity.
Allow all the rest of the header to be included, regardless of the
setting of EFI_LOADER.

With the inclusion of blk.h the 'struct blk_desc' declaration is
unnecessary. Drop it while we are here.

Signed-off-by: Simon Glass <sjg@chromium.org>