include/kernel: add custom USER/DOMAIN config options

These allow the generated kernel's build metadata to be defined explicitly.
This metadata is reported, eg, at boot time and in `uname -a` on running
systems. If the variables aren't configured, the current build system username
and hostname are used as normal.

The motivation for this option is to achive reproducible (bit-for-bit
identical) kernel builds of official openwrt releases.

Signed-off-by: bryan newbold <bnewbold@robocracy.org>
Signed-off-by: Felix Fietkau <nbd@openwrt.org>

SVN-Revision: 48541