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>