preerase: Inverted LFS3_PREERASE to be opt-in

The original motivation for making LFS3_PREERASE opt-out, is that it
makes sense for LFS3_GBMAP to bring in all gbmap-related features
(PREERASE, BADBLOCKS (future)). However, after a bit of use, I think
this just complicates our ifdef logic too much.

So instead, LFS3_PREERASE is now opt-in, with the intention of making
all ifdefs relative only to the default build. I think this will make it
easier to reason about ifdefs, at least internally.

Eventually, I want to look into alternative default builds (LFS3_BIGGER,
LFS3_BIGGERR, ..., LFS3_BIGGEST), which would provide an alternative way
to enable all gbmap-related features. Though these builds have a
high-risk of bikeshedding (LFS3_GC?), so we'll see.

---

That being said, the main ergonomic improvement was probably adding
a #error, so we don't have to check ifdef GBMAP everywhere.

Maybe this should be extended to LFS3_RDONLY? Or maybe not, LFS3_RDONLY
is a bit of a special case.

No code changes:

                    code          stack          ctx
  before:          35144           2136          660
  after:           35144 (+0.0%)   2136 (+0.0%)  660 (+0.0%)

                    code          stack          ctx
  gbmap before:    38380           2144          776
  gbmap after:     38380 (+0.0%)   2144 (+0.0%)  776 (+0.0%)

                    code          stack          ctx
  preerase before: 38920           2168          796
  preerase after:  38920 (+0.0%)   2168 (+0.0%)  796 (+0.0%)

lfs3_util.h

@@ -50,6 +50,9 @@
 #ifndef LFS3_GBMAP
 #define LFS3_GBMAP
 #endif
+#ifndef LFS3_PREERASE
+#define LFS3_PREERASE
+#endif
 #ifndef LFS3_BLEAFCACHE
 #define LFS3_BLEAFCACHE
 #endif
@@ -83,6 +86,9 @@
 #ifdef LFS3_YES_GBMAP
 #define LFS3_GBMAP
 #endif
+#ifdef LFS3_YES_PREERASE
+#define LFS3_PREERASE
+#endif
 #ifdef LFS3_YES_BLEAFCACHE
 #define LFS3_BLEAFCACHE
 #endif
@@ -103,6 +109,12 @@
 #endif
 #endif
 
+// some define dependencies
+#if defined(LFS3_PREERASE) \
+        && (!defined(LFS3_GBMAP) || !defined(LFS3_REVPERTURB))
+#error "LFS3_PREERASE requires LFS3_GBMAP and LFS3_REVPERTURB"
+#endif
+
 
 // System includes
 #include <stdint.h>
@@ -313,15 +325,15 @@
 #define LFS3_IFYES_GBMAP(a, b, c) (c)
 #endif
 
-#ifndef LFS3_NO_PREERASE
+#ifdef LFS3_PREERASE
 #define LFS3_IFDEF_PREERASE(a, b) (a)
 #else
 #define LFS3_IFDEF_PREERASE(a, b) (b)
 #endif
 
-#if !defined(LFS3_NO_PREERASE) && defined(LFS3_YES_PREERASE)
+#if defined(LFS3_PREERASE) && defined(LFS3_YES_PREERASE)
 #define LFS3_IFYES_PREERASE(a, b, c) (a)
-#elif !defined(LFS3_NO_PREERASE)
+#elif defined(LFS3_PREERASE)
 #define LFS3_IFYES_PREERASE(a, b, c) (b)
 #else
 #define LFS3_IFYES_PREERASE(a, b, c) (c)