build: fix regression for kernels < 5.10

build: fix regression for kernels < 5.10

This fixes a regression introduced with commit
5ed1e5140a80558ab47fd70410ae3242bed5becf ("build: build kernel image
before building modules/packages").

Before this commit the make target would always include "modules",
resulting in a MODPOST and a complete Module.symvers file. Since this
commit a MODPOST of the kernel modules is not guaranteed for kernels <
5.10. This results in some broken SDKs in which external packages that
depend on exported symbols from kernel modules fail to compile.

Adding "modules" back to the calls to the CompileImage defines fixes the
regression. For kernels > 5.10 this is not needed, but it doesn't cause
any harm either.

Tested with kernels 5.4.x and 5.10.x.

Signed-off-by: Sebastian Kemper <sebastian_ml@gmx.net>