build: allow for reproducible builds

build: allow for reproducible builds

Setting and using the SOURCE_DATE_EPOCH variable takes
care of most of the magic necessary.
https://reproducible-builds.org/docs/source-date-epoch/

vpp-ext-deps packages after this change is being built with that
date set to date of the last modification of the
subtree (similar logic to deriving the "number" for
the package version)

For the rest of the packages, pinning the following
three variables should result in bit-identical
artifacts across multiple runs:

export SOURCE_DATE_EPOCH=$(date +%s)
export VPP_BUILD_HOST="buildhost"
export VPP_BUILD_USER="builduser"

Add a blurb in the docs describing this new functionality.

Type: improvement
Change-Id: I71b085f0577b2358aa98f01dafd8e392239420a6
Signed-off-by: Andrew Yourtchenko <ayourtch@gmail.com>