kernel-devsrc: use the common NI kernel-devsrc.inc
kernel-devsrc: use the common NI kernel-devsrc.inc

The kernel-devsrc.bbappend currently uses its own implementation of the
NI-specific postinst methods. There already exists a
kernel-devsrc-nilrt.inc file which includes a more modern version of the
same function, and which is sourced by the alternative kernel-devsrc
packages.

Use the kernel-devsrc-nilrt.inc file which is common to all NILRT
kernel-devsrc implementations, in the primary version of the package,
for easier maintenance.

Signed-off-by: Alex Stewart <04cdb2793e4fb0120cc30d1c823099f3862701f4@ni.com>
(cherry picked from commit 623d332ea2d5eee310eea4a618327bbe1c1c2a5d)

ni/meta-nilrt,ni/meta-nilrt,ni/meta-nilrt,ni/meta-nilrt,ni/meta-nilrt