kernel:config:mx6: boot time optimization

requested review from @marc-oliver.westerburg and removed review request for @jonas.hoeppner

Hi Marc, can you have a look on side effects of these kernel configs. It decreases the boot time by about 10 sec (30 to 20). I don't see we loose any functionality used in a released product (so might need to reenable things for debug).

I agree with you for the most part, that these settings can be disabled for our regular production kernels - especially if this saves 10s of boot-time.

With one exception: This patch also disbles CONFIG_GPIO_SYSFS!

While this the GPIO_SYSFS interface seems to be deprecated (https://www.thegoodpenguin.co.uk/blog/stop-using-sys-class-gpio-its-deprecated/) AFAIK this is still the standard way we use and document to our customers to access GPIO pins. The new perfered interface (CONFIG_GPIOLIB) seems to have been integrated with kernel 4.8 only fairly recently - considering our OSses- and therefore isn't even available in our Rocko und earlier Yocto releases.

I guess we should still keep CONFIG_GPIO_SYSFS enabled.

Hi @marc-oliver.westerburg, hi @jonas.hoeppner,

When CONFIG_EXPERT is disabled, CONFIG_GPIO_SYSFS is not visible. I've prepared a MR to make it available without expert mode: seco-ne/kernel/linux-guf!277 (merged)

I have also noticed that a lot of unused i.MX platforms are enabled in defconfig. To reduce image size and boot time, it is better to disable them (please check the new patch in this MR).

The MR to make CONFIG_GPIO_SYSFS visible again seems simple enough and is probably way simpler than leaving CONFIG_EXPERT enabled and instead making all other changed done by this option by hand. Should be fine.

The other i.MX-platforms can indeed be disabled. While we do have i.MX31, i.MX35, and i.MX53 designs, we no longer support any of these in kirkstone. All the other i.MX-platforms were never enabled on purpose by us. Probably got enabled due to merging with NXP's defconfigs at some time.