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/bcm958622hr_defconfig b/configs/bcm958622hr_defconfig
index 67e18b6..62e50b6 100644
--- a/configs/bcm958622hr_defconfig
+++ b/configs/bcm958622hr_defconfig
@@ -19,6 +19,7 @@
 CONFIG_CMD_FAT=y
 # CONFIG_MMC is not set
 CONFIG_SYS_NS16550=y
+CONFIG_FAT_WRITE=y
 CONFIG_SHA1=y
 CONFIG_SHA256=y
 CONFIG_OF_LIBFDT=y