ps1: bundle plain-ELF POPS loader + KEYS.BIN (iPoPS, MIT)

Fix 80020148 'game could not be started' on ARK-5: the base PBP's encrypted
~PSP loader fails to link on modern CFW. Use the bundled plain-ELF loader
(data/loader/DATA.PSP) as the DATA.PSP section instead, and write the 16-byte
NPDRM version key (data/loader/KEYS.BIN) alongside the EBOOT so CFW can
decrypt the STARTDAT PGD. Both assets are from iPoPS (MIT, attributed).

- load_data_psp: prefer the bundled ELF, fall back to the base ~PSP loader.
- load_keys_bin: return the bundled 16-byte key when present.
- convert: write KEYS.BIN next to the output EBOOT.
- README + data/loader/ATTRIBUTION.md: document the loader and its license.

data/loader/ATTRIBUTION.md

@@ -0,0 +1,30 @@
+# Loader assets
+
+The files in this directory are bundled from [iPoPS](https://github.com/julianxhokaxhiu/iPoPS)
+by Julian Xhokaxhiu, licensed under the MIT License:
+
+- `DATA.PSP` — a plain-ELF PSP POPS loader (relocatable PRX, 19002 bytes).
+- `KEYS.BIN` — a 16-byte NPDRM version key used by CFW to decrypt the
+  `STARTDAT` PGD.
+
+MIT License
+
+Copyright (c) 2020 Julian Xhokaxhiu
+
+Permission is hereby granted, free of charge, to any person obtaining a copy of
+this software and associated documentation files (the "Software"), to deal in
+the Software without restriction, including without limitation the rights to
+use, copy, modify, merge, publish, distribute, sublicense, and/or sell copies
+of the Software, and to permit persons to whom the Software is furnished to do
+so, subject to the following conditions:
+
+The above copyright notice and this permission notice shall be included in all
+copies or substantial portions of the Software.
+
+THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
+SOFTWARE.