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/boot/bless-boot.c

@@ -5,6 +5,7 @@
 
 #include "alloc-util.h"
 #include "bootspec.h"
+#include "efi-loader.h"
 #include "efivars.h"
 #include "fd-util.h"
 #include "fs-util.h"