make it easy to change kernel config options with pmbootstrap

Right now the process for adjusting the kernel config to satisfy the required options is the following:

• run pmbootstrap kconfig check
• run pmbootstrap kconfig edit
• navigate to eachoption that needs to be changed in the UI and change it manually (takes some time!)
• save it

A much better process would be:

• one pmbootstrap command / argument, that just fixes up a kernel config automatically
• (additionally one pmbootstrap command to change one kernel option quickly)

We can't simply change one line from commented out to "= y" in the kernel config file, because that does not take care of dependencies.

But merge_config.sh can do it. It's a short script, maybe we should just copy it to the pmbootstrap source dir (to pmb/data) so we ensure we have the same version for all the crazy kernel trees we work with.