Lots of folks are starting to leverage hybrid FPGA/ARM chips, like the Xilinx Zynq series. Folks who work with these typically get pretty involved in both embedded development and RTL, as part of the challenge is figuring out what segment of compute lives where in those types of systems.

There are plenty of places who'd take a decent firmware engineer and then allow them to learn the gateware portion through absorption.

A master's in this area also is a good way to break in. Defense companies tend to value it pretty highly.