boot: Simplify object erasure

This erase_obj() machinery looks like voodoo and creates an awful lot of
noise as soon as we get back to building with -O0. We can do this in a
more simple way by introducing a struct that holds the information we
need on cleanup. When building with optimization enabled, all this gets
inlined and the eraser vanishes.

src/basic/memory-util.c

@@ -38,21 +38,3 @@ bool memeqbyte(uint8_t byte, const void *data, size_t length) {
         /* Now we know first 16 bytes match, memcmp() with self.  */
         return memcmp(data, p + 16, length) == 0;
 }
-
-#if !HAVE_EXPLICIT_BZERO
-/*
- * The pointer to memset() is volatile so that compiler must de-reference the pointer and can't assume that
- * it points to any function in particular (such as memset(), which it then might further "optimize"). This
- * approach is inspired by openssl's crypto/mem_clr.c.
- */
-typedef void *(*memset_t)(void *,int,size_t);
-
-static volatile memset_t memset_func = memset;
-
-void* explicit_bzero_safe(void *p, size_t l) {
-        if (l > 0)
-                memset_func(p, '\0', l);
-
-        return p;
-}
-#endif