shared: split out ESP/XBOOTLDR search stuff from bootspec.c

The code is quite different from the rest of bootspec.c, with different
deps and stuff. There's even a /***/ line to separate the two parts.
Given how large the file already is, let#s just split it into two.

No code changes, just some splitting out.

src/boot/bootctl.c

@@ -24,6 +24,7 @@
 #include "escape.h"
 #include "fd-util.h"
 #include "fileio.h"
+#include "find-esp.h"
 #include "fs-util.h"
 #include "glyph-util.h"
 #include "main-func.h"