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-generator.c

@@ -4,7 +4,7 @@
 #include <sys/stat.h>
 #include <unistd.h>
 
-#include "efivars.h"
+#include "efi-loader.h"
 #include "generator.h"
 #include "log.h"
 #include "mkdir.h"

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"

src/boot/bootctl.c

@@ -24,6 +24,7 @@
 #include "bootspec.h"
 #include "copy.h"
 #include "dirent-util.h"
+#include "efi-loader.h"
 #include "efivars.h"
 #include "env-util.h"
 #include "escape.h"