package/boost: enable building of boost_log

Signed-off-by: Yannick Kiekens <yannickkiekens@gmail.com>
[Thomas: tested on ARM uClibc, and AArch64 glibc, the latter being the
case that used to fail building, and was the reason why boost log had
been disabled.]
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>