Add support for pogoplug v4 devices

This uses a slightly tweaked uboot and now uses FDT as well.
Move Sun RPC support from package to libc

Except in the case of musl libc, in which case, continue to use
the uclibc librpc package.
Add config symbols for kernel keyring support
Don't assume that 9999 is high enough
Break out libcom_err and libss from e2fsprogs

Also make these tools available on the host during build