UBUNTU: [Packaging] check debian.env to determine do_libc_dev_package

The linux-libc-dev package provides UAPI headers. The Linux kernel
promises backward compatibile API to userspace. That is the reason
why there exists only one linux-lib-dev per series. Its package name
does not contain any ABI number or flavour string since there is no
point in installing multiple instances of linux-lib-dev.

This leads to an obvious conclusion; only the master kernel should
provide linux-libc-dev.

Currently, it is checked at compile-time. If you attempt to build
linux-libc-dev on a non-master kernel, it errors out with
"non-master branch building linux-libc-dev, aborting".

When constructing debian/control, the linux-libc-dev enablement is
determined by do_libc_dev_package, which is set to true if the
'variants' variable contains '--'. This is tricky, and it is difficult
to understand the intention. In fact, do_libc_dev_package is true for
most kernels except linux-unstable. Derivative kernels disable
linux-libc-dev by removing linux-libc-dev.stub.

This commits adopts a simpler logic; enable linux-libc-dev if
debian/debian.env contains DEBIAN=debian.master.

Please note this commit changes the behaviour of linux-unstable.
linux-unstable previously disabled linux-lib-dev because it defined
variants=-wip, but it is enabled now. It should not be a big deal
since we do not publish linux-unstable.

Signed-off-by: Masahiro Yamada <masahiro.yamada@canonical.com>
Acked-by: Dimitri John Ledkov <dimitri.ledkov@canonical.com>
Signed-off-by: Dimitri John Ledkov <dimitri.ledkov@canonical.com>

debian/rules

@@ -173,10 +173,8 @@ include $(DROOT)/rules.d/4-checks.mk
 
 control_files := $(DEBIAN)/control.stub.in
 ifeq ($(do_libc_dev_package),true)
-ifneq (,$(wildcard $(DEBIAN)/control.d/linux-libc-dev.stub))
 	control_files += $(DEBIAN)/control.d/linux-libc-dev.stub
 endif
-endif
 
 # Calculate Ubuntu Compatible Signing levels
 UBUNTU_COMPATIBLE_SIGNING=$(shell grep -qx ' *Subject: C = GB, ST = Isle of Man, O = Canonical Ltd., OU = Secure Boot, CN = Canonical Ltd. Secure Boot Signing (2021 v3)' debian/canonical-revoked-certs.pem && echo ubuntu/4 pro/3)

debian/rules.d/0-common-vars.mk

@@ -121,9 +121,7 @@ ifeq ($(do_full_build),false)
 do_source_package_content=false
 endif
 
-# linux-libc-dev may not be needed, default to building it only for the
-# primary variant
-ifneq ($(filter --,$(variants)),)
+ifeq ($(DEBIAN),debian.master)
 do_libc_dev_package=true
 else
 do_libc_dev_package=false

debian/rules.d/2-binary-arch.mk

@@ -557,10 +557,6 @@ binary-arch-headers: install-arch-headers
 	dh_testdir
 	dh_testroot
 ifeq ($(do_libc_dev_package),true)
-ifeq ($(filter debian.master%,$(DEBIAN)),)
-	echo "non-master branch building linux-libc-dev, aborting"
-	exit 1
-endif
 	$(call dh_all,linux-libc-dev)
 endif