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/test/test-boot-timestamps.c

@@ -5,7 +5,7 @@
 
 #include "acpi-fpdt.h"
 #include "boot-timestamps.h"
-#include "efivars.h"
+#include "efi-loader.h"
 #include "log.h"
 #include "tests.h"
 #include "util.h"

src/test/test-condition.c

@@ -14,7 +14,7 @@
 #include "cgroup-util.h"
 #include "condition.h"
 #include "cpu-set-util.h"
-#include "efivars.h"
+#include "efi-loader.h"
 #include "hostname-util.h"
 #include "id128-util.h"
 #include "ima-util.h"