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/colibri_imx6_defconfig b/configs/colibri_imx6_defconfig
index fcb7c53..d8307b0 100644
--- a/configs/colibri_imx6_defconfig
+++ b/configs/colibri_imx6_defconfig
@@ -49,6 +49,7 @@
 CONFIG_G_DNL_MANUFACTURER="Toradex"
 CONFIG_G_DNL_VENDOR_NUM=0x1b67
 CONFIG_G_DNL_PRODUCT_NUM=0x4000
+CONFIG_FAT_WRITE=y
 CONFIG_OF_LIBFDT=y
 CONFIG_OF_LIBFDT_OVERLAY=y
 # CONFIG_EFI_LOADER is not set