package/tvheadend: unbreak

tvheadend tries to impersonate an autotools package, when it is not.
This breaks the build.

tvheadend's ./configure tries to run compiled code.
This breaks the build.

Switch tvheadend over to using the generic-package infrastructure,
and provide carefully-crafted _CMDS for each steps.

We still keep a post-install hook to install our "DB" since it is not
part of tvheadend, but an addition of our own packaging of tvheadend.

Fixes:
    http://autobuild.buildroot.net/results/3ea/3eaa17b94975477263fe12e201eec9cbeeb2912c/
    http://autobuild.buildroot.net/results/7c8/7c85db0dfab98808c97a5ef17dec9d1071dc0d49/


and a few others as well.

[Peter: tweak 004-dont-run-compiled-code.patch description as pointed out by Thomas]
Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Cc: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>