syscalls: Scan multiple folders to build complete syscall list

Previously the syscall list was generated only from the include
folder. This is a limitation when the application tries to create
system calls. This patch create a simple way to include these
new syscalls without the application touching the kernel.

This can be enabled by a Kconfig CONFIG_APPLICATION_DEFINED_SYSCALL.
Once enabled the application source directory will be scanned to
find all application defined syscalls.

Signed-off-by: Adithya Baglody <adithya.nagaraj.baglody@intel.com>