Split libnu into libplatform, libamex

Going to add some ALLS stuff to libplatform and the ALLS hardware
does not contain an AMEX board.

platform/meson.build

@@ -0,0 +1,18 @@
+platform_lib = static_library(
+    'platform',
+    include_directories : inc,
+    implicit_include_directories : false,
+    c_pch : [
+        '../precompiled.c',
+        '../precompiled.h',
+    ],
+    dependencies : [
+        capnhook.get_variable('hook_dep'),
+    ],
+    sources : [
+        'hwmon.c',
+        'hwmon.h',
+        'nusec.c',
+        'nusec.h',
+    ],
+)