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/am335x_igep003x_defconfig b/configs/am335x_igep003x_defconfig
index fa468f0..651e194 100644
--- a/configs/am335x_igep003x_defconfig
+++ b/configs/am335x_igep003x_defconfig
@@ -49,6 +49,7 @@
 CONFIG_MMC_OMAP_HS=y
 CONFIG_MTD_UBI_FASTMAP=y
 CONFIG_SYS_NS16550=y
+CONFIG_FAT_WRITE=y
 CONFIG_OF_LIBFDT=y
 CONFIG_FDT_FIXUP_PARTITIONS=y
 # CONFIG_GENERATE_SMBIOS_TABLE is not set