[yocto] Proper way of rebuilding kernel image

[Michael Hu]

Hi,

I am compiling Yocto kernel image with:
* a third party's "in-tree" kernel configuration
* the same third party's BSP (which has some further kernel configurations
as well)
* my own kernel configuration fragments for my project on top of the above
two

What I found was if I make a small change to my own kernel config fragment,
e.g. just add an empty line or add a comment, *don't *do a bitbake clean,
and bitbake the kernel image again, the "in-tree" kernel configuration
doesn't get picked up, therefore the new kernel's .config file is
drastically different than a clean build.

But If I do a bitbake clean kernel recipe and bitbake the kernel image
again, I get the same .config file both times.

The way I specify "in-tree" kernel configuration:
KBUILD_DEFCONFIG_machine_name ?= "<machine_name>_defconfig"
KCONFIG_MODE_machine_name ?="--alldefconfig"

What's the proper way of rebuilding kernel image once recipe has changes?
Seems like doing a cleanall build every time takes a lot of time and
inefficient.

Thanks!
.M.
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