blk: Support block drivers in TPL

At present it is not possible to enable/disable block drivers in TPL. This
is needed to provide sandbox support. Add a Kconfig option and adjust the
Makefile.

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

drivers/block/Kconfig

@@ -27,6 +27,18 @@ config SPL_BLK
 	  be partitioned into several areas, called 'partitions' in U-Boot.
 	  A filesystem can be placed in each partition.
 
+config TPL_BLK
+	bool "Support block devices in TPL"
+	depends on TPL_DM && BLK
+	default y
+	help
+	  Enable support for block devices, such as SCSI, MMC and USB
+	  flash sticks. These provide a block-level interface which permits
+	  reading, writing and (in some cases) erasing blocks. Block
+	  devices often have a partition table which allows the device to
+	  be partitioned into several areas, called 'partitions' in U-Boot.
+	  A filesystem can be placed in each partition.
+
 config BLOCK_CACHE
 	bool "Use block device cache"
 	depends on BLK

drivers/block/Makefile

@@ -11,4 +11,4 @@ endif
 
 obj-$(CONFIG_IDE) += ide.o
 obj-$(CONFIG_SANDBOX) += sandbox.o
-obj-$(CONFIG_$(SPL_)BLOCK_CACHE) += blkcache.o
+obj-$(CONFIG_$(SPL_TPL_)BLOCK_CACHE) += blkcache.o