I want to change the <PKG>_CONF_OPTS config options for a package included in Buildroot without altering the baseline <pkg>.mk file. Based on what I'm reading, it shouldn't be necessary to completely override the package using the BR2_PACKAGE_OVERRIDE_FILE as I don't want to change the source or anything else significant.
It has been suggested that this can be done using BR2_EXTERNAL and external.mk, but I cannot find any examples and my attempts have failed. The Buildroot documentation is quite silent on this topic, as near as I can tell, and instead only explains the creation of new packages rather than the build customization of included packages.
I have tried adding lines to external.mk directly as in <PKG>_CONF_OPTS += --new-option but this has no effect. I have also tried creating a supplementary makefile in package/<pkg>/<pkg>.mk and filling in the needed information in the Config.in files for a "new" package but this has also failed.
I have referenced a couple of related questions that do not have complete answers given, so I am hoping to discover the general method and document it here.
How to pass extra custom ./configure autotools options to a Buildroot package?
Buildroot: Build only one package as both shared and static lib, all others shared only
Hopefully with a couple hints we can get this figured out and then documented for others.
EDIT: Done quite a bit of testing with this, and it's interesting to note that even the "sanity check" case of making the changes to _CONF_OPTS directly in the existing package is not having the desired effect. In my case (trying to do a static linkage build of one particular package while leaving everything else dynamic) I find that the presence or absence of BR2_STATIC_LIBS in the _defconfig controls everything when it comes down to the final build command, despite acknowledgement to the contrary all throughout the build logs.
It would appear that there is no satisfactory way to accomplish this. After going through the make process in more detail, I find that the top level defconfig BR2_ options override the individual package options (such as, in my case, the presence or absence of BR2_STATIC_LIBS).