replace kernel_autoconf with sel4_autoconf

since the mainline kernel's kernel_autoconf doesn't include
seL4/gen_config.h but sel4_autoconf does

Trivial: Style fix

make the style checker happy

link kernel_autoconf explicitly

explicitly link kernel_autoconf on each subdirectory

seL4_libs: specify linking method

this commit set as much private linked config as possible

seL4_libs: remove autoconf.h from seL4_libs

this commit removes autoconf.h generation from each lib and
append '#include <<lib_name>/gen_config.h>' after each
'#include <autoconf.h>' since autoconf.h is only the for the
kernel config now. This is a temporarily solution, since we
currently don't have a way to tell which header files each file
is trying to include when it includes 'autoconf.h'

serialserver: Remove sel4test library dependency

libsel4test is only required as a library for the tests.

sel4serialserver: Create separate lib for tests

Placing the tests in a separate library so that they don't need to get
built in projects that don't need them.

CMake: Mark config options as advanced

This is to hide the options in the cmake-gui. Top level applications can
choose to reexpose these options by clearing the advanced setting.

trivial: style CMakeLists.txt files

Fix serialserver config definition

output clients in different colors

So they can be distinguished in output. This is enabled by default but
can be turned off with a config option.

libsel4serialserver: Fix build dependencies

SELFOUR-1107 move serial server to own library