Convert to SPDX license tags

Signed-off-by: Gerwin Klein <gerwin.klein@data61.csiro.au>

replace kernel_autoconf with sel4_autoconf

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

explicitly link configs

explicitly link kernel_autoconf for subprojects

sel4_proj_libs: remove global configuration lib

this commit removes autoconf.h generation from sel4_proj_libs 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'

Add libsel4rpc

libsel4rpc is intended to provide a 'default' interface
for managing resource allocation requests across an endpoint.