scripts: move gen_syscalls.py to scripts/build

Move scripts needed by the build system and not designed to be run
individually or standalone into the build subfolder.

Signed-off-by: Anas Nashif <anas.nashif@intel.com>
diff --git a/CMakeLists.txt b/CMakeLists.txt
index cd38de5..8260fd1 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -734,7 +734,7 @@
   # Also, some files are written to include/generated/syscalls/
   COMMAND
   ${PYTHON_EXECUTABLE}
-  ${ZEPHYR_BASE}/scripts/gen_syscalls.py
+  ${ZEPHYR_BASE}/scripts/build/gen_syscalls.py
   --json-file        ${syscalls_json}                     # Read this file
   --base-output      include/generated/syscalls           # Write to this dir
   --syscall-dispatch include/generated/syscall_dispatch.c # Write this file