Allow skipping the build of kernel-headers package

When a newer kernel branch is published into the official repositories,
its kernel-headers, cpupower, perf anf kernel-doc packages will be used
instead. One could turn off building of the last three, but there was no
such switch for kernel-headers. Add it.

kernel.spec

@@ -57,6 +57,9 @@
 %define build_devel 		1
 %define build_debug		0
 
+# Build kernel-headers package
+%define build_headers		1
+
 # Build nrj desktop kernels
 %define build_nrj_desktop		1
 
@@ -537,6 +540,7 @@ Conflicts:	%{_lib}cpufreq-devel
 This package contains the development files for cpupower.
 %endif
 
+%if %{build_headers}
 %package headers
 Version:	%kversion
 Release:	%rpmrel
@@ -559,6 +563,7 @@ should use the 'kernel-devel' package instead.
 %if %{build_cpupower}
 %exclude %_includedir/cpufreq.h
 %endif
+%endif
 
 #
 # End packages - here begins build stage
@@ -692,8 +697,10 @@ BuildKernel() {
 	%smake INSTALL_MOD_PATH=%{temp_root} KERNELRELEASE=$KernelVer modules_install
 
 	# headers
+	%if %{build_headers}
 		%make INSTALL_HDR_PATH=%{temp_root}%{_prefix} KERNELRELEASE=$KernelVer headers_install
 		find %{temp_root}%{_prefix} -name .install -or -name ..install.cmd | %kxargs rm -f
+	%endif
 
 	# remove /lib/firmware, we use a separate kernel-firmware
 	rm -rf %{temp_root}/lib/firmware