boost: enable back log on powerpc with uclibc

boost-log builds fine with powerpc on uclibc nowadays so enable it back.
By removing this dependency, build failure on azmq is also fixed as this
package is currently selecting boost-log without fulfilling this
dependency

Fixes:
 - http://autobuild.buildroot.net/results/9c373d0b5a1a59e2271d71c480d55a90a67b84cb



Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>