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/ls1021aiot_qspi_defconfig b/configs/ls1021aiot_qspi_defconfig
index 3deda48..3628367 100644
--- a/configs/ls1021aiot_qspi_defconfig
+++ b/configs/ls1021aiot_qspi_defconfig
@@ -4,7 +4,7 @@
 CONFIG_SYS_EXTRA_OPTIONS="QSPI_BOOT"
 CONFIG_BOARD_EARLY_INIT_F=y
 CONFIG_CMD_GPT=y
-CONFIG_DOS_PARTITION=y
+CONFIG_CMD_FAT=y
 CONFIG_OF_CONTROL=y
 CONFIG_DM=y
 CONFIG_FSL_CAAM=y