remoteproc: core: Task sync during rproc_fw_boot()

By default, rproc_fw_boot() needs to wait for rproc to be configured,
but a race may occur when using rpmsg/virtio.  In this case, it can
be called locally in a safe manor.

This patch represents two usecases:

 - External call (via exported rproc_boot()), which waits
 - Internal call can use 'nowait' version of rproc_boot()

Signed-off-by: Fabrice Gasnier <fabrice.gasnier@st.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>
Signed-off-by: Bjorn Andersson <bjorn.andersson@linaro.org>