fs: Add support for a filesystem

The existing filesystem support is fairly basic. It supports access to
only one filesystem at a time, does not allow caching, does not have an
equivalent of Linux's VFS and uses global variables to keep track of
which one is in use.

As a starting point to improving this, provide a filesystem uclass. For
now it only includes operations to mount and unmount.

Provide a bootdev so that it is possible to locate bootflows on a
filesystem.

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

test/boot/bootstd_common.h

@@ -24,10 +24,11 @@
 enum {
 	HUNTER_ETH	= 0,
 	HUNTER_SIMPLE_BUS,
-	HUNTER_MMC	= 3,
-	HUNTER_SCSI	= 6,
-	HUNTER_USB	= 8,
-	HUNTER_COUNT	= 10,
+	HUNTER_FS	= 3,
+	HUNTER_MMC,		/* ID of MMC hunter */
+	HUNTER_SCSI	= 7,	/* ID of SCSI hunter */
+	HUNTER_USB	= 9,	/* ID of USB hunter */
+	HUNTER_COUNT	= 11,
 	HUNTER_MAX	= HUNTER_COUNT - 1,
 };