First, we have not specified which GPL version is applicable, or if there are specific exemptions to it. Here is the applicable section under GPLv1:
"Source code for a work means the preferred form of the work for making
modifications to it. For an executable file, complete source code means
all the source code for all modules it contains; but, as a special
exception, it need not include source code for modules which are standard
libraries that accompany the operating system on which the executable
file runs, or for standard header files or definitions files that
accompany that operating system."
Nothing there about build scripts or tools to build.... Let's take a look at GPLv3 (section 1):
"The “Corresponding Source” for a work in object code form means all the source code needed to generate, install, and (for an executable work) run the object code and to modify the work, including scripts to control those activities. However, it does not include the work's System Libraries, or general-purpose tools or generally available free programs which are used unmodified in performing those activities but which are not part of the work. For example, Corresponding Source includes interface definition files associated with source files for the work, and the source code for shared libraries and dynamically linked subprograms that the work is specifically designed to require, such as by intimate data communication or control flow between those subprograms and other parts of the work."
I see the section regarding build scripts, but embedded devices often do not include the ability to build on the device itself, presenting a big difference from the desktop world where you would typically config, build, install all from source. I still think that simply including the settings/defines/prerequisites would suffice. Note that the section above also specifically excludes general purpose tools. Since installation is often accomplished either through external flashing device or embedded flash utility (general purpose tool), these would be excluded.
Yeah, I've read the GPL. I'm not sure I like where things are going in v2 and v3. On the embedded devices that I work with (for instance FreeRTOS), the licenses are GPLv1 for the overall OS and tasking system. They also include a specific exemption for custom tasks/routines which doesn't require you to release unless you change the OS.
Ultimately, it boils out to defining which device, which specific license and which specific files are involved. None of that's been done, so to assume the most radicalized position here doesn't make sense.