REGRESSION: kernel: configure the kernel only when an input changed

[Russell Senior]

https://github.com/openwrt/openwrt/commit/e97e3f36bd5d48add83c5c7fba907600b12a653c

This commit results in a regression on realtek switches (and perhaps
elsewhere), where a stale initramfs kernel gets included in a
subsequent squashfs image. See explanation here:
https://pastebin.com/xhHsTQng. Reverting this commit was tested and
fixes the problem.

To reproduce, with a .config stub of:

  CONFIG_TARGET_realtek=y
  CONFIG_TARGET_realtek_rtl931x=y
  CONFIG_TARGET_realtek_rtl931x_DEVICE_zyxel_xs1930-12hp=y

time make BUILD_LOG=1 IGNORE_ERRORS=m V=s target/linux/clean
time make BUILD_LOG=1 IGNORE_ERRORS=m V=s
$ ls -al bin/targets/realtek/rtl931x/*.bin
<touch a file in the files overlay>
time make -j1 BUILD_LOG=1 IGNORE_ERRORS=m V=s
$ ls -al bin/targets/realtek/rtl931x/*.bin

The squashfs-sysupgrade.bin file size will jump by a few megabytes in
the second build and, more significantly, boots as an initramfs
instead of a squashfs due to the kernel-embdedded cpio rootfs.

-- 
Russell Senior
russell at personaltelco.net