headers: Fix headers guards

Any word started with underscore followed by and uppercase letter or a
second underscore is a reserved word according with C99.

With have *many* violations on Zephyr's code, this commit is tackling
only the violations caused by headers guards. It also takes the
opportunity to normalize them using the filename in uppercase and
replacing dot with underscore. e.g file.h -> FILE_H

Signed-off-by: Flavio Ceolin <flavio.ceolin@intel.com>
diff --git a/scripts/gen_syscalls.py b/scripts/gen_syscalls.py
index ad90629..e385f94 100755
--- a/scripts/gen_syscalls.py
+++ b/scripts/gen_syscalls.py
@@ -24,8 +24,8 @@
 
 list_template = """
 /* auto-generated by gen_syscalls.py, don't edit */
-#ifndef _ZEPHYR_SYSCALL_LIST_H_
-#define _ZEPHYR_SYSCALL_LIST_H_
+#ifndef ZEPHYR_SYSCALL_LIST_H
+#define ZEPHYR_SYSCALL_LIST_H
 
 %s
 
@@ -45,7 +45,7 @@
 
 #endif /* _ASMLANGUAGE */
 
-#endif /* _ZEPHYR_SYSCALL_LIST_H_ */
+#endif /* ZEPHYR_SYSCALL_LIST_H */
 """
 
 syscall_template = """