Refactor build scripts to have common code in a common script file

[janvorli]

Currently, the build.sh, init-tools.sh, build-packages.sh and build-test.sh contains copies of the same code e.g. for host rid detection. We should move the shared code to a shared shell script file so that we have just one copy of that code.
Later on, it would be ideal to share that code even between all of our repos.

[janvorli]

cc: @eerhardt

[4creators]

This same is true for cmd scripts: build.cmd, build-test.cmd, run.cmd, init-tools.cmd and tests\runtest.cmd all have duplicated functionality. Perhaps it would be good to do the same for windows scripts as well.

[jkotas]

FWIW, CoreRT solved this problem by having the common setup in https://github.com/dotnet/corert/blob/master/buildscripts/buildvars-setup.cmd and https://github.com/dotnet/corert/blob/master/buildscripts/buildvars-setup.sh; and calling this script as the first thing from the main script.

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