linux-image-extra is special, it is only additive to linux-image, this
means really cannot use the standard kernel postinst/postrm for this
package.  As it also depends on linux-image we know that linux-image will
have been installed before it, and will be removed after it.  On change
(installation/update/removal) of linux-image-extra we want to run the
kernel postinst to rebuilt the initramfs and update the bootloader
as necessary.  To this end switch to package specific postinst/postrm
which trigger the /etc/kernel/postinst.d hooks.  We need to do it this
way to get the specially parameterised incantations of update-initramfs,
to ensure we trigger the correct build rather than mearly dpkg triggering
a rebuild of the running kernel.

BugLink: http://bugs.launchpad.net/bugs/1375310Signed-off-by: Andy Whitcroft <apw@canonical.com>