Makefile: finish DESTDIR support

Support originally added in 0f97427a38cd0c2c2aee00946ebe561453165a50.
I'm sure this was left in accidentally as it resets the variable to ''.
Merge destdir-support-for-make-install (#7) into ifixit-production

Makefile: finish DESTDIR support

Support originally added in 0f97427a38cd0c2c2aee00946ebe561453165a50.
I'm sure this was left in accidentally as it resets the variable to ''.