Bug#1117999: dpkg-buildflags: enable ELF package note metadata

I don't like the approach that is taken here.

- If the build system of a package misses passing flags,
   then you don't embed that info into the binaries.

- The four linkers (BFD ld, gold, mold and lld) also
   take that data from the environment, and there is
   also precedence for taking such data from the
   environment, like the TIMESTAMP for the build.

- dpkg-buildflags with the proposed implementation
   causes autopkg test regressions with the additional
   stderr output (already six regressions with the
   tests triggered by dpkg).  There will be a lot
   more ...

So please set that in the environment instead, it seems to be the more 
robust approach.

Matthias