externalsrc: Work with reproducible_build

Externalsrc removes do_fetch, do_unpack, and do_patch.  The system normally
discovers the correct reproducible date as a postfuncs of do_unpack, so this
date is never found, so it falls back to the default epoch.

Instead we can move the discovery function to a prefuncs on the epoch
deploy task.  This task will run before do_configure, and since the source
is already available can run safely at anytime.

Signed-off-by: Mark Hatle <mark.hatle@xilinx.com>
Signed-off-by: Mark Hatle <mark.hatle@kernel.crashing.org>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>