Make small bootloader for chainloading other payloads

[webhdx]

As first mentioned in #22 ideally PicoBoot would be shipped with minimal bootloader that will read actual payload from memory first. This let's provide visual confirmation that the modchip was installed correctly and any other potential issues are coming from somewhere else (i.e. SD card setup). It is also important to keep control over boot process if someone decides to use other IPL replacement apps (iplboot or cubeboot). By utilizing payload checksum it can even work as a simple recovery mode.