kboot: use 128MB for the heap area, ZFS needs a lot of memory

ZFS uses a lot of memory. The old minimal allocations won't work when
ZFS support is added. Most environments this will be used (or will
liekly be used) have >> 256MB, 128MB should be safe everywhere and allow
examination of a fair number of ZFS pools to boot from.

Sponsored by:		Netflix

stand/kboot/main.c

@@ -150,7 +150,7 @@ int
 main(int argc, const char **argv)
 {
 	void *heapbase;
-	const size_t heapsize = 15*1024*1024;
+	const size_t heapsize = 128*1024*1024;
 	const char *bootdev;
 
 	archsw.arch_getdev = kboot_getdev;