autotools.bbclass: use ordinary append for file-checksums update

Commit 29daffc2 added an
anonymous python function to append the site file dependencies
for do_configure. This causes any recipe that inherits allarch
after autotools to end up with a different set of file checksums
for each machine, since the anonymous python would get processed
before the TARGET_ARCH changes in allarch.

Use an ordinary += assignment to fix this.

Signed-off-by: Matt Madison <matt@madison.systems>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>