[SRU zesty/xenial/trusty] LP#1705495 -- limit rebuild test duration

[Andy Whitcroft]

We have recently had problems with the kernel rebuild test failing in
timeout on armhf.  This test is intended to show that toolchain changes do
not prevent the kernel building.  Right now this performs a full kernel
build for the architecure.  This is excessive as a single flavour build
would validate this.  If we look at the existing code we added a new
build profile for autopkgtest use which would reduce the flavours to one.
We have never used it.  It should also be noted that the name of the
profile variable has changed since this was penned and it was never
updated.

Following this email is a patch for zesty, xenial and trusty.  Firstly
fixes the naming of the profile variable.  Secondly it uses the profile
when doing rebuild tests.  Note that this uses the short form -P<profile>
form for compatibility back to trusty.

Proposing for SRU to zesty, xenial, and trusty.

-apw