coreos-kernel: update to 3.17.6

No changes in config:

```patch
--- files/x86_64_defconfig-3.17.2	2014-12-07 17:39:05.335669466 -0500
+++ files/x86_64_defconfig-3.17.6	2014-12-07 17:57:30.895718797 -0500
@@ -1,6 +1,6 @@
 #
 # Automatically generated file; DO NOT EDIT.
-# Linux/x86 3.17.2 Kernel Configuration
+# Linux/x86 3.17.6 Kernel Configuration
 #
 CONFIG_64BIT=y
 CONFIG_X86_64=y
@@ -2918,7 +2918,6 @@
 # CONFIG_RTC_DRV_DS1553 is not set
 # CONFIG_RTC_DRV_DS1742 is not set
 # CONFIG_RTC_DRV_DS2404 is not set
-# CONFIG_RTC_DRV_EFI is not set
 # CONFIG_RTC_DRV_STK17TA8 is not set
 # CONFIG_RTC_DRV_M48T86 is not set
 # CONFIG_RTC_DRV_M48T35 is not set
```