glibc/eglibc has getent built automatically, it should be copied to target root filesystem for package ecryptfs to run. uclibc has a getent under extra/scripts which emulates getenv in glibc/eglibc. there might be other packages need getent, which is small in size, I would suggest always put that to target rootfs?
A solution for this problem was proposed at http://lists.busybox.net/pipermail/buildroot/2014-August/104410.html.
getent support was merged in http://git.buildroot.net/buildroot/commit/package/getent?id=db4919bcac12ff9d2383dd6c69e3042e0658247a and http://git.buildroot.net/buildroot/commit/package/ecryptfs-utils?id=66215ffc6d711840180ad1fdba1548d037c05bdf. Bug is fixed.