Speedup "make disk_image" by allowing parallel build

[reticulatedpines]

"make" in platform/ will use -j16 etc, but only within one cam, these are deliberately serialised to avoid race conditions (rather than fix the actual bugs, which is a different question...).

"make disk_image" is quite slow (needs to spin up a qemu vm to mount the disk image) and should be safe to parallelise. It depends on the zip for each cam, so it should be possible to keep the cam builds serial, and trigger a parallel disk_image job at the end.

See platform/Makefile:

 19 # => use .NOTPARALLEL in the upper-level Makefiles only
 20 # parallel build is still used within each platform
 21 .NOTPARALLEL:
 22 #