Electrical Engineering background, jack of all trades:
Hardware - ASIC, FPGA (VHDL, Verilog, etc.) - uControllers (MIPS, ARM, PIC, etc.) - Analog/digital circuit design (Orcad, spice, etc.)
Software - Embedded C (TCP/IP, embedded OS, hardware drivers) - Server side: PHP, Node.JS, MySQL - Client side: Javascript, JQuery, Angular, ...
Plus all the other things that startups do: product development, sales, marketing, documentation (Framemaker, Photoshop, Illustrator), web site...