util-lib: split shared/efivars into basic/efivars and shared/efi-loader

I want to use efivars.[ch] in proc-cmdline.c, but most of the efivars stuff is
not needed in basic/. Move the file from shared/ to basic/, but then move back
most of the higher-level functions to the new shared/efi-loader.c file.

src/shared/bootspec.c

@@ -15,6 +15,7 @@
 #include "device-nodes.h"
 #include "dirent-util.h"
 #include "efivars.h"
+#include "efi-loader.h"
 #include "env-file.h"
 #include "env-util.h"
 #include "fd-util.h"