procd: add SELinux support

procd: add SELinux support

This commit adds a patch to procd to support loading the SELinux
policy early at boot time, and adjusts the procd package to use this
SELinux support when libselinux is enabled.

The procd patch has been submitted separately [1]: obviously the
intent is to have it merged in the procd Git repository rather than
have it in OpenWrt itself.

[1] http://lists.infradead.org/pipermail/openwrt-devel/2019-November/025791.html

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>
[rebase, add commit message]
Signed-off-by: W. Michael Petullo <mike@flyn.org>
[split commit into openwrt.git and procd.git]
Signed-off-by: Daniel Golle <daniel@makrotopia.org>