I finally got it, Secure boot MUST be disabled, on one of the tabs in bios you must scroll down to find this option, then after power on i got asus splash screen and tapping F2 into bios and all boot options are now available, hope this helps!
I am having this same issue with an asus x551 that i bought on ebay with a cracked digitizer. It did not come with an hdd so i pulled a working one from my desktop. I have tried booting from disk as well as a flash drive, but it immediately goes back to bios everytime, in boot device options it...