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/am43xx_evm_ethboot_defconfig b/configs/am43xx_evm_ethboot_defconfig
index 6a30706..20874dd 100644
--- a/configs/am43xx_evm_ethboot_defconfig
+++ b/configs/am43xx_evm_ethboot_defconfig
@@ -58,4 +58,5 @@
 CONFIG_G_DNL_MANUFACTURER="Texas Instruments"
 CONFIG_G_DNL_VENDOR_NUM=0x0403
 CONFIG_G_DNL_PRODUCT_NUM=0xbd00
+CONFIG_FAT_WRITE=y
 CONFIG_OF_LIBFDT=y