Kconfig: Migrate FS_FAT / FAT_WRITE

Now that these symbols are in Kconfig, migrate all users.  Use imply on
a number of platforms that default to having this enabled.  As part of
this we must migrate some straglers for CMD_FAT and DOS_PARTITION.

Signed-off-by: Tom Rini <trini@konsulko.com>
diff --git a/configs/ds109_defconfig b/configs/ds109_defconfig
index 46ac567..d8f373b 100644
--- a/configs/ds109_defconfig
+++ b/configs/ds109_defconfig
@@ -12,7 +12,7 @@
 CONFIG_CMD_DHCP=y
 CONFIG_CMD_PING=y
 CONFIG_CMD_DATE=y
-CONFIG_DOS_PARTITION=y
+CONFIG_CMD_FAT=y
 CONFIG_ISO_PARTITION=y
 # CONFIG_MMC is not set
 CONFIG_SPI_FLASH=y