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: 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'

sel4test: add RPC support

This just adds a few members to sel4test structs that enable use of
sel4rpc in sel4test.

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

sel4test: Declare env struct formally in libsel4test

This environment definition is presently used by any test declared by DEFINE_TEST, and so
must be a common declaration.

For correct compilation the partial (and incorrect) duplicate of the environment in the
serialserver tests is removed in this same commit

sel4test: simplify output buffering

- remove BUFFER_OUTPUT, as PRINT_XML implies it
- use config_set where possible
- replace interface with the following:
sel4test_start_printf_buffer: start buffering output
sel4test_end_printf_buffer: dump output and stop buffering
- remove a lot of ifdefs. sel4test is not build for performance
and its
best we compile as much code as possible.
sel4test_start_printf_buffer
does nothing if PRINT_XML is not enabled.

Add CMake build rules for various libraries