rpiboot always installs to /usr/..., should allow setting the prefix

[bringert]

Describe the bug

Most software allows the user installation to select an installation prefix when building from source. Commonly this is /usr/local. rpiboot/Makefile has hardcoded installation into /usr.

Steps to reproduce the behaviour

• Follow the build instructions at https://github.com/raspberrypi/usbboot/blob/master/Readme.md#building
• Install with make install

Observed: The code is installed under /usr.

Expected: Some way to direct it to install under /usr/local without having to edit the Makefile

Device(s)

Other

Compute Module IO board.

No response

RPIBOOT logs

No response

Kernel logs

No response

Device UART logs

No response