Qemuppc Boot Hangs

qemuppc is randomly hanging at boot during sanity testing. For example:

• https://autobuilder.yocto.io/builders/nightly-ppc-lsb/builds/581
• https://autobuilder.yocto.io/builders/nightly-ppc/builds/592

Despite initial suspicions this was not due to entropy problems.

It is *not*:

• Specific to a host OS (observed on centos7, debian8, ubuntu1710)
• Specific to image recipe (happens sato, sato-sdk, lsb, lsb-sdk)
• Specific to DISTRO (poky and poky-lsb)

The hang usually occurs after:

[    6.667445] udevd[104]: starting version 3.2.2
[    6.743262] udevd[105]: starting eudev-3.2.2

and is hanging in the poll() call in udevadm-settle.c:adm_settle(), it doesn't seem to return from the syscall.

In some cases the boot proceeds further and hangs in the dropbear key creation.