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>
diff --git a/drivers/block/blk-uclass.c b/drivers/block/blk-uclass.c
index 21c5209..1a6e8f8 100644
--- a/drivers/block/blk-uclass.c
+++ b/drivers/block/blk-uclass.c
@@ -743,8 +743,7 @@
 
 static int blk_post_probe(struct udevice *dev)
 {
-	if (CONFIG_IS_ENABLED(PARTITIONS) &&
-	    IS_ENABLED(CONFIG_HAVE_BLOCK_DEVICE)) {
+	if (CONFIG_IS_ENABLED(PARTITIONS) && blk_enabled()) {
 		struct blk_desc *desc = dev_get_uclass_plat(dev);
 
 		part_init(desc);