opkg: save output of postinst scripts when image first boot

opkg: save output of postinst scripts when image first boot

When image feature "debug-tweaks" enabled, save output of postinstall
scripts to log file /var/log/postinstall.log when image first boot. And the
log file can be configured.

It also needs image feature "package-management" enabled. If not,
package run-postinsts will be installed and then all packages will be
configured by it. Command 'opkg configure' outputs nothing.

[YOCTO #3223]

(From OE-Core rev: 378504c1c0ab1e0e09950b37effc04221b928236)

Signed-off-by: Kang Kai <kai.kang@windriver.com>
Signed-off-by: Saul Wold <sgw@linux.intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>