env: ENV_IS_IN_FAT improvements

Make it select FS_FAT as well, because if it's not selected, enabling
ENV_IS_IN_FAT causes a Kconfig warning:

warning: (ENV_IS_IN_FAT) selects FAT_WRITE which has unmet direct dependencies (FS_FAT)

This also allows dropping some code from config_fallbacks.

Also drop the unnecessary help text about having to enable
CONFIG_FAT_WRITE - Kconfig automatically handles that.

Signed-off-by: Tuomas Tynkkynen <tuomas@tuxera.com>
diff --git a/env/Kconfig b/env/Kconfig
index 692f863..a243707 100644
--- a/env/Kconfig
+++ b/env/Kconfig
@@ -74,13 +74,11 @@
 config ENV_IS_IN_FAT
 	bool "Environment is in a FAT filesystem"
 	depends on !CHAIN_OF_TRUST
+	select FS_FAT
 	select FAT_WRITE
 	help
 	  Define this if you want to use the FAT file system for the environment.
 
-	  - CONFIG_FAT_WRITE:
-	  This must be enabled. Otherwise it cannot save the environment file.
-
 config ENV_IS_IN_EXT4
 	bool "Environment is in a EXT4 filesystem"
 	depends on !CHAIN_OF_TRUST