Kconfig: rename BOOT_MEDIA_SPI_BUS to BOOT_DEVICE_SPI_FLASH_BUS

Provide a default value of 0 in drivers/spi as there weren't
default values aside from specific mainboards and arch/x86.
Remove any default 0 values while noting to keep the option's
default to 0.

BUG=chrome-os-partner:56151

Change-Id: If9ef585e011a46b5cd152a03e41d545b36355a61
Signed-off-by: Aaron Durbin <adurbin@chromium.org>
Reviewed-on: https://review.coreboot.org/16192
Tested-by: build bot (Jenkins)
Reviewed-by: Furquan Shaikh <furquan@google.com>
Reviewed-by: Martin Roth <martinroth@google.com>
diff --git a/src/mainboard/google/gru/Kconfig b/src/mainboard/google/gru/Kconfig
index d5733c8..3150839 100644
--- a/src/mainboard/google/gru/Kconfig
+++ b/src/mainboard/google/gru/Kconfig
@@ -67,7 +67,7 @@
 	hex
 	default 5
 
-config BOOT_MEDIA_SPI_BUS
+config BOOT_DEVICE_SPI_FLASH_BUS
 	int
 	default 1