No longer possible to check out the version that works with QEMU master

[sorear]

I know this isn't the intended use of this repository but the recent rebasing on sifive/riscv-linux make git submodule update fail for sifive/freedom-u-sdk@5d38ed:

fatal: reference is not a tree: 3989911d0071b93d2e529d22007b7ec783a57532
Unable to checkout '3989911d0071b93d2e529d22007b7ec783a57532' in submodule path 'linux'

(5d38ed is the last priv-1.9 freedom-u-sdk; the current rev is priv-1.9.1, which is not supported by qemu.)

[a0u]

I have restored commit sifive/riscv-linux@3989911 as tag freedom-u-sdk-20161128.

[sorear]

Thanks! It builds now.