src/poky:python2.7: improve reproducibility

The compiled .pyc files contain time stamp corresponding to the compile time.
This prevents binary reproducibility. This patch allows to achieve binary
reproducibility by overriding the build time stamp by the value
exported via SOURCE_DATE_EPOCH.

Patch by Bernhard M. Wiedemann, backported from python/cpython#296

[YOCTO#11241]

(From OE-Core rev: 2a044f1e4f5c63e11e631b31f741c7aabfa6f601)

Signed-off-by: Juro Bystricky <juro.bystricky@intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>