Make linux kernel builds reproducible when BUILDBOT selected

The linux kernel is not reproducible because the build user
and domain is included into the kernel. Set the build user
to `builder` and build domain to buildhost.

It's also possible to build reproducible builds by setting
KERNEL_BUILD_USER KERNEL_BUILD_DOMAIN to static values.

Signed-off-by: Alexander Couzens <lynxis@fe80.eu>

config/Config-kernel.in

@@ -6,6 +6,7 @@
 
 config KERNEL_BUILD_USER
 	string "Custom Kernel Build User Name"
+	default "builder" if BUILDBOT
 	default ""
 	help
 	  Sets the Kernel build user string, which for example will be returned
@@ -14,6 +15,7 @@ config KERNEL_BUILD_USER
 
 config KERNEL_BUILD_DOMAIN
 	string "Custom Kernel Build Domain Name"
+	default "buildhost" if BUILDBOT
 	default ""
 	help
 	  Sets the Kernel build domain string, which for example will be