classes/buildhistory: save preinst/postinst/prerm/postrm

classes/buildhistory: save preinst/postinst/prerm/postrm

Write the value of these package script variables into the packageinfo
so that any changes to them can be tracked (in separate files since they
are multi-line).

Inspired by an earlier patch from Andreas Müller <schnitzeltony@googlemail.com>

Signed-off-by: Paul Eggleton <paul.eggleton@linux.intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>