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/zynq_zc770_xm012_defconfig b/configs/zynq_zc770_xm012_defconfig
index 16cd613..943efdb 100644
--- a/configs/zynq_zc770_xm012_defconfig
+++ b/configs/zynq_zc770_xm012_defconfig
@@ -1,6 +1,7 @@
 CONFIG_ARM=y
 CONFIG_ARCH_ZYNQ=y
 CONFIG_SYS_TEXT_BASE=0x4000000
+# CONFIG_SPL_FAT_SUPPORT is not set
 CONFIG_DEFAULT_DEVICE_TREE="zynq-zc770-xm012"
 CONFIG_FIT=y
 CONFIG_FIT_SIGNATURE=y