deb: allow custom dpkg command

deb: allow custom dpkg command

trying to reduce the turnaround times for local builds, i'm trying to
find a way to inject `-Zgzip -z1` into the dkpg-deb command line.

attached patch introduces a `DPKG_BUILDCMD` variable for this purpose.
(i'm not very familiar with the whole bitbake infrastructure, so i'm
wondering: do i have to add it to the `vardeps` of do_package_deb?)

tia,
tim

>From 62d4930d307d5d07844889001e8a1c3111b72b98 Mon Sep 17 00:00:00 2001
From: Tim Blechmann <tim@klingt.org>
Date: Thu, 8 Aug 2019 06:48:54 +0200
Subject: [PATCH] package_deb: allow dpkg-deb to be customized via
 DPKG_BUILDCMD

the command line to invoke `dpkg-deb` is hardcoded. there are certain use
cases where we want to tweak how debian packages are compiled: e.g. the
default uses xz compression, which is rather CPU intensive. for local
builds one might want to pass `-Zgzip -z1` to favor speed over compression
ratio.

we therefore introduce a `DPKG_BUILDCMD` variable which allows downstream
code to customize how dpkg-deb is executed

(From OE-Core rev: 4e00ceaf38f2ae338e3b192c3485aaa963f54b99)

Signed-off-by: Tim Blechmann <tim@klingt.org>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>