[File] support for Android ELF notes

[enh]

i added support for decoding the Android ELF note to toybox file(1)
some time ago, and it's pretty useful for any doing Android
development: you can see what version of Android the binary requires,
and -- for binaries built by the NDK rather than as part of the OS
itself -- what version of the NDK was used.

i knocked up a quick patch in
https://gist.github.com/enh/d7538e32aca8b68ed3541dd803594b9d (which
i'll also paste below, but that'll doubtless lose the tabs thanks to
gmail) but i'm assuming that it's not okay to just interpret strings
directly as strings, and further than you already have helper
functions i should use for that (rather than write my own)?

please point me at something i can copy & paste my way into trouble from :-)

diff --git a/src/readelf.c b/src/readelf.c
index a79906f5..6fd877ec 100644
--- a/src/readelf.c
+++ b/src/readelf.c
@@ -686,6 +686,27 @@ do_os_note(struct magic_set *ms, unsigned char
*nbuf, uint32_t type,
  return -1;
  return 1;
  }
+
+ if (NAMEEQUALS(name, "Android") && type == 1 && descsz >= 4) {

(oh, this was the other question i had --- do you want me to #define
the single-use NT_ANDROID_TYPE_IDENT constant for the `1` here, or do
you prefer the literal?)

+   uint32_t api_level;
+   *flags |= FLAGS_DID_OS_NOTE;
+   memcpy(&api_level, &nbuf[doff], sizeof(api_level));
+   api_level = elf_getu32(swap, api_level);
+   if (file_printf(ms, ", for Android %d", api_level) == -1)
+     return -1;
+   /*
+   * NDK r14 and later also include details of the NDK that
+   * built the binary. OS binaries (or binaries built by older
+   * NDKs) don't have this. The NDK release and build number
+   * are both 64-byte strings.
+   */
+   if (descsz >= 4+64+64) {
+     if (file_printf(ms, ", built by NDK %.64s (%.64s)",

(this is the bit where i'm assuming you might want more checking than
just `%.64s`?)

+         &nbuf[doff+4], &nbuf[doff+4+64]) == -1)
+       return -1;
+   }
+ }
+
  return 0;
 }