boost: fix do_fetch failure

Bintray service has been discontinued causing boost do_fetch to fail:
WARNING: boost-1.76.0-r0 do_fetch: Failed to fetch URL
https://dl.bintray.com/boostorg/release/1.76.0/source/boost_1_76_0.tar.bz2

,
attempting MIRRORS if available

Signed-off-by: Stefan Ghinea <stefan.ghinea@windriver.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>