Any solutions yet? I am having the same problem with my Dell Vostro 3546.
And external keyboards are not working!
Also, the dell diagnostic tool doesn't show any errors.
I was having the same problem: I could enter the boot options (F12) screen, but the keyboard was not responsive, and thus arrow keys, esc, enter had no effect. Solved by entering BIOS setup (F2) and enabling:
Advanced -> Miscellaneous Devices -> USB debug
Hope that helps!