There are several guides for kernel developers and users. These guides can be rendered in a number of formats, like HTML and PDF. Please read Documentation/admin-guide/README.rst first.
In order to build the documentation, use make htmldocs or
make pdfdocs. The formatted documentation can also be read online at:
https://www.kernel.org/doc/html/latest/
There are various text files in the Documentation/ subdirectory, several of them using the Restructured Text markup notation.
Please read the Documentation/process/changes.rst file, as it contains the requirements for building and running the kernel, and information about the problems which may result by upgrading your kernel.
I'm working on writing extensions, fixing bugs and make the optimize the source for a better source handling as developer. A few additional commands are:
Someone not knowing from where comes the header <gelf.h> or <openssl/opensslv.h> or similar, type make dep_install to install all required dependencies. If you're not using apt, then search instead for libelf-dev and libssl-dev or on other distros they may be libssl-devel and libelf-devel.
To install the kernel and modules together, type make res_install to install them all to the new directory _install, which can be used as root directory. Note that there will only be installed the kernel and the modules. In the skeleton you can then add a userland via busybox or similar.
To remove the _install directory, type make res_cleaning.
Just use this source code unter the terms of the Linux licenses, as known. The additional extensions are also under GPL.
Maybe there are a few developers wondering why the kernel searches for headers like gelf.h or openssl/opensslv.h. For this you must install on Linux the packages libelf-dev and libssl-dev or similar on you linux distro. Then all these compilation errors disapear.