pmb/parse/_apkbuild: Fix locating kernel subpackages with "-" in name

When defining a new kernel subpackage with a "-" in it (e.g. $pkgname-kernel-mainline-modem), then pmbootstrap is unable to find the function that builds the subpackage:

ERROR: Could not find subpackage function, no line starts with 'kernel_mainline-modem() {'

This is because it assumes that a $pkgname-kernel- subpackage is built by a kernel_ function, but this does not have to be the case.

We should really respect the name of the subpackage function that is specified when defining the subpackage, but unfortunately it is stripped away in cut_off_function_names().

For now let's fix this by replacing "-" with "_", but ideally the APKBUILD parser API should be refactored to expose the subpackage function in the future.