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/sama5d2_ptc_spiflash_defconfig b/configs/sama5d2_ptc_spiflash_defconfig
index 21014b9..8771e08 100644
--- a/configs/sama5d2_ptc_spiflash_defconfig
+++ b/configs/sama5d2_ptc_spiflash_defconfig
@@ -17,6 +17,7 @@
 # CONFIG_CMD_FLASH is not set
 CONFIG_CMD_SF=y
 # CONFIG_CMD_FPGA is not set
+CONFIG_CMD_FAT=y
 # CONFIG_MMC is not set
 CONFIG_SPI_FLASH=y
 CONFIG_USB=y