Added config for FAT label (espressif#178)

This activates the code for f_setlabel() and f_getlabel. These functions should take up less than 1k of symbol space.
Once these are available, I will add one-liner functions to the FFat and SD libraries to set/get volume name. I'm working on an example which exposes an SD card through the USBMSC, and being able to set the name would be a very nice touch.

Author: lbernstone

configs/defconfig.common

@@ -36,6 +36,7 @@ CONFIG_FATFS_CODEPAGE_850=y
 CONFIG_FATFS_LFN_STACK=y
 # CONFIG_FATFS_API_ENCODING_ANSI_OEM is not set
 CONFIG_FATFS_API_ENCODING_UTF_8=y
+CONFIG_FATFS_USE_LABEL=y
 # CONFIG_FMB_CONTROLLER_SLAVE_ID_SUPPORT is not set
 CONFIG_FMB_TIMER_PORT_ENABLED=y
 CONFIG_FREERTOS_HZ=1000