build: namespace the generated headers with `zephyr/`

Namespaced the generated headers with `zephyr` to prevent
potential conflict with other headers.

Introduce a temporary Kconfig `LEGACY_GENERATED_INCLUDE_PATH`
that is enabled by default. This allows the developers to
continue the use of the old include paths for the time being
until it is deprecated and eventually removed. The Kconfig will
generate a build-time warning message, similar to the
`CONFIG_TIMER_RANDOM_GENERATOR`.

Updated the includes path of in-tree sources accordingly.

Most of the changes here are scripted, check the PR for more
info.

Signed-off-by: Yong Cong Sin <ycsin@meta.com>
diff --git a/kernel/userspace_handler.c b/kernel/userspace_handler.c
index da4053b..ab6e4f0 100644
--- a/kernel/userspace_handler.c
+++ b/kernel/userspace_handler.c
@@ -64,7 +64,7 @@
 				    object));
 	k_thread_perms_set(ko, thread);
 }
-#include <syscalls/k_object_access_grant_mrsh.c>
+#include <zephyr/syscalls/k_object_access_grant_mrsh.c>
 
 static inline void z_vrfy_k_object_release(const void *object)
 {
@@ -74,16 +74,16 @@
 	K_OOPS(K_SYSCALL_VERIFY_MSG(ko != NULL, "object %p access denied", object));
 	k_thread_perms_clear(ko, _current);
 }
-#include <syscalls/k_object_release_mrsh.c>
+#include <zephyr/syscalls/k_object_release_mrsh.c>
 
 static inline void *z_vrfy_k_object_alloc(enum k_objects otype)
 {
 	return z_impl_k_object_alloc(otype);
 }
-#include <syscalls/k_object_alloc_mrsh.c>
+#include <zephyr/syscalls/k_object_alloc_mrsh.c>
 
 static inline void *z_vrfy_k_object_alloc_size(enum k_objects otype, size_t size)
 {
 	return z_impl_k_object_alloc_size(otype, size);
 }
-#include <syscalls/k_object_alloc_size_mrsh.c>
+#include <zephyr/syscalls/k_object_alloc_size_mrsh.c>