Make some CI test code optional
    
    The only present test case rebuilt, installed and booted the kernel.
    Now let's make the install and boot steps optional, just on explicit
    request. Skipping these steps will be the default behaviour, at least
    for now.
    
    You can override the default by setting variable "JUST_BUILD" in the
    environment the test runs in. The variable defaults to "yes" which
    means the test will just build the kernel. If you use JUST_BUILD=no
    (or JUST_BUILD=false or JUST_BUILD=0) then the test will not only
    build but install and boot the kernel, too.
    
    Note that the acceptable values of JUST_BUILD are case insensitive.