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/at91sam9rlek_mmc_defconfig b/configs/at91sam9rlek_mmc_defconfig
index 9a40c41..edf9881 100644
--- a/configs/at91sam9rlek_mmc_defconfig
+++ b/configs/at91sam9rlek_mmc_defconfig
@@ -42,3 +42,4 @@
 CONFIG_DEBUG_UART_ANNOUNCE=y
 CONFIG_ATMEL_USART=y
 CONFIG_LCD=y
+CONFIG_FAT_WRITE=y