Grouped explicit targets (the &: syntax) indicate to Make that a single
recipe, built once, produces all the indicated targets. In this case,
the feature was being used to indicate that the kernel build also
creates the needed dtbs for the sunxi devices.
Unfortunately, this feature was only introduced in GNU make 4.3 on
January 27 2020. Most current or LTS (or current LTS) releases of
distributions don't have this version of Make yet. For example, Ubuntu
20.04, Fedora 32, and Debian 10 (Buster) are still on GNU make 4.2 or
older. When grouped targets are used on older versions of GNU make,
it tries to run the same recipe once for each target. In this case, a
'make all' at the top level would run 'make -C src/linux ...' three
times. This caused file locking problems when anything but 'make -j1'
was used and made build times pretty long in any other case.
Fix this by changing the dtbs to dependency-only targets. Now GNU make
<4.3 correctly intuits "I only need to build kernel-sunxi.gz once and
will provide the dtbs, too" instead of "I need to build this
kernel-sunxi.gz recipe three times: once for kernel-sunxi.gz, once for
the two dtbs"
