chrony: bump to version 2.2

chrony: bump to version 2.2

With the new version, documentation is not generated part of the main
build. This removes the need to alter the Makefile to remove the invoke
on the `chrony.txt` target. Cleanup also explicitly disabling seccomp
(since its experimental) and tomcrypt (Buildroot does not have
libtomcrypt at this time).

[Thomas: use tabs for indentation in the definition of
CHRONY_CONF_OPTS.]

Signed-off-by: James Knight <james.knight@rockwellcollins.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>