bdk: Use Kconfig options instead of getenv()

* Use Kconfig options instead of unusable getenv
* Select CAVIUM_BDK_DDR_TUNE_HW_OFFSETS on CN81XX
* Fix Coverity CID 1393976 (DEADCODE)

Tested on Cavium's cn8100_sff_evb.

Change-Id: Ia16c0161b0e9cf5d06418e46556c0fb45532a5b1
Signed-off-by: Patrick Rudolph <patrick.rudolph@9elements.com>
Reviewed-on: https://review.coreboot.org/27448
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Martin Roth <martinroth@google.com>
Reviewed-by: Philipp Deppenwiese <zaolin.daisuki@gmail.com>

src/soc/cavium/cn81xx/Kconfig

@@ -11,6 +11,7 @@ config SOC_CAVIUM_CN81XX
 	select HAVE_MONOTONIC_TIMER
 	select UART_OVERRIDE_REFCLK
 	select SOC_CAVIUM_COMMON
+	select CAVIUM_BDK_DDR_TUNE_HW_OFFSETS
 
 if SOC_CAVIUM_CN81XX