kbuild: deb-pkg improve maintainer address generation

There is multiple issues with the genaration of maintainer string

It uses DEBEMAIL and EMAIL enviroment variables, which may contain angle brackets,
creating invalid maintainer strings. The documented KBUILD_BUILD_USER and
KBUILD_BUILD_HOST variables are not used. Undocumented and uncommon NAME
variable is used. Refactor the Maintainer string to:

- use EMAIL or DEBEMAIL directly if they are in form "name <user@host>"
- use KBUILD_BUILD_USER and KBUILD_BUILD_HOST if set before falling
  back to autodetection
- no longer use NAME variable or the useless Anonymous string

The logic is switched from multiline if/then/fi statements to compact
shell variable substition commands.

Reported-by: Mathieu Malaterre <malat@debian.org>
Signed-off-by: Riku Voipio <riku.voipio@linaro.org>
Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>