deb-pkg: generate correct build dependencies

bison/flex is now needed always for building for kconfig. Some build
dependencies depend on kernel configuration, enable them as needed:

- libelf-dev when UNWINDER_ORC is set
- libssl-dev for SYSTEM_TRUSTED_KEYRING

Since the libssl-dev is needed for extract_cert binary, denote with
:native to install the libssl-dev for the build machines architecture,
rather than for the architecture of the kernel being built.
Tested-by: Manivannan Sadhasivam <manivannan.sadhasivam@linaro.org>
Signed-off-by: Riku Voipio <riku.voipio@linaro.org>
Reviewed-by: Ben Hutchings <ben@decadent.org.uk>
Acked-by: maximilian attems <maks@stro.at>
[masahiro.yamada: change 'flex' to 'flex | flex:native' ]
Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>

scripts/package/mkdebian

@@ -134,6 +134,8 @@ fi
 
 mkdir -p debian/
 echo $debarch > debian/arch
+extra_build_depends=", $(if_enabled_echo UNWINDER_ORC libelf-dev)"
+extra_build_depends="$extra_build_depends, $(if_enabled_echo SYSTEM_TRUSTED_KEYRING libssl-dev:native)"
 
 # Generate a simple changelog template
 cat <<EOF > debian/changelog
@@ -170,7 +172,7 @@ Source: $sourcename
 Section: kernel
 Priority: optional
 Maintainer: $maintainer
-Build-Depends: bc, kmod, cpio
+Build-Depends: bc, kmod, cpio, bison, flex | flex:native $extra_build_depends
 Homepage: http://www.kernel.org/
 
 Package: $packagename