discover: Fix unused function warning

discover: Fix unused function warning

clang errors out about an unused have_busybox function:

 discover/paths.c:44:13: error: unused function 'have_busybox' [-Werror,-Wunused-function]
 static bool have_busybox(void)
            ^

Move have_busybox() to inside the #ifndef PETITBOOT_TEST scope to
eliminate the warning and avoid having #ifdefs in load_url_async().

Signed-off-by: Joel Stanley <joel@jms.id.au>
Signed-off-by: Samuel Mendoza-Jonas <sam@mendozajonas.com>
	[Moved definition into #ifndef PETITBOOT_TEST instead of using
	 #ifdef at the call site]