env: add absolute path at CONFIG_ENV_EXT4_FILE

Add the absolute path to the default value of
CONFIG_ENV_EXT4_FILE = "/uboot.env".

This patch avoid the error :
  Saving Environment to EXT4... File System is consistent
  Please supply Absolute path
Reviewed-by: NTom Rini <trini@konsulko.com>
Signed-off-by: NPatrick Delaunay <patrick.delaunay@st.com>

env/Kconfig

@@ -479,7 +479,7 @@ config ENV_EXT4_DEVICE_AND_PART
 config ENV_EXT4_FILE
 	string "Name of the EXT4 file to use for the environment"
 	depends on ENV_IS_IN_EXT4
-	default "uboot.env"
+	default "/uboot.env"
 	help
 	  It's a string of the EXT4 file name. This file use to store the
 	  environment (explicit path to the file)