fs: ext4: Fix building ext4 in SPL if write is enabled

If EXT4_WRITE is enabled, write capabilities will be compiled into SPL, but
not CRC16. Add an option to enable CRC16 to avoid linker errors.

Signed-off-by: Sean Anderson <seanga2@gmail.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
diff --git a/lib/Kconfig b/lib/Kconfig
index 79cf9ef..f6ca559 100644
--- a/lib/Kconfig
+++ b/lib/Kconfig
@@ -687,6 +687,12 @@
 	  checksum with feedback to produce an 8-bit result. The code is small
 	  and it does not require a lookup table (unlike CRC32).
 
+config SPL_CRC16
+	bool "Support CRC16 in SPL"
+	depends on SPL
+	help
+	  Enables CRC16 support in SPL. This is not normally required.
+
 config CRC32
 	def_bool y
 	help