build-unit-test-docker: simplify fetching

Most packages we use come from github, except for boost.  Create a
'pkg_url' function to get the URL for a revision for curl calls.
This function automatically creates the URL from the package name,
unless the package has a 'url' property.  If the 'url' property exists,
treat it as a function that generates the URL from the package name and
revision (to handle the boost case and any future non-github packages).

Signed-off-by: Patrick Williams <patrick@stwcx.xyz>
Change-Id: Ifc47a1b580e6ec3728d8deeaa4c568017a80e03f
1 file changed