blk: Use a function for whether block devices are available

blk: Use a function for whether block devices are available

At present we use HAVE_BLOCK_DEVICE to indicate when block devices are
available.

This is a very strange option, since it partially duplicates the BLK
option used by driver model. It also covers both U-Boot proper and SPL,
even though one might have block devices and another not.

As a first step towards correcting this, create a new inline function
called blk_enabled() which indicates if block devices are available.
This cannot be used in Makefiles, or #if clauses, but can be used in C
code.

A function is useful because we cannot use CONFIG_IS_ENABLED(BLK) to
decide if block devices are needed, since we must consider the legacy
block interface, enabled by HAVE_BLOCK_DEVICE

Update a few places where it can be used and drop some unnecessary #if
checks around some functions in disk/part.c - rely on the compiler's
dead-code elimination instead.

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