mtdblock: Add comment about UBI block devices

There is a surprisingly large number of tutorials
that suggest using mtdblock to mount SquashFS filesystems
on flash devices, including NAND devices.

Given this approach is suboptimal than using UBI, and given
the UBI block device layer was introduced many years ago
specifically with this use case in mind, add a small
comment inviting users and developers to consider UBI block.

Signed-off-by: Ezequiel Garcia <ezequiel@collabora.com>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Link: https://lore.kernel.org/linux-mtd/20210801234509.18774-7-ezequiel@collabora.com

drivers/mtd/Kconfig

@@ -69,6 +69,9 @@ config MTD_BLOCK_RO
 	  You do not need this option for use with the DiskOnChip devices. For
 	  those, enable NFTL support (CONFIG_NFTL) instead.
 
+comment "Note that in some cases UBI block is preferred. See MTD_UBI_BLOCK."
+	depends on MTD_BLOCK || MTD_BLOCK_RO
+
 config FTL
 	tristate "FTL (Flash Translation Layer) support"
 	depends on BLOCK