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>

env/Kconfig

@@ -74,13 +74,11 @@ config ENV_IS_IN_EEPROM
 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

include/config_fallbacks.h

@@ -29,10 +29,6 @@
 #define CONFIG_SYS_BAUDRATE_TABLE	{ 9600, 19200, 38400, 57600, 115200 }
 #endif
 
-#if defined(CONFIG_ENV_IS_IN_FAT) && !defined(CONFIG_FS_FAT)
-#define CONFIG_FS_FAT
-#endif
-
 /* Rather than repeat this expression each time, add a define for it */
 #if defined(CONFIG_IDE) || \
 	defined(CONFIG_SATA) || \