feat: add AmberELEC hardware targets

AmberELEC's metapackage adds LIBRETRO_CORES_EXTRA (bsnes, Mesen, melonDS, geolith) only on RG552, RG353 and RG503, and deletes those cores from es_systems.cfg on every other device (packages/amberelec/package.mk:23-31, 45-58 at a21975bf). The new target scraper reads the Makefile's device images, that test and es_systems.cfg the way the platform scraper does: RG351P, RG351V and RG351MP list 125 cores, the other three 136. The installer reads the device from COREELEC_DEVICE in os-release (scripts/image:168).

install.py

@@ -109,7 +109,7 @@ TARGETS_URL = f"{BASE_URL}/install/targets/{{platform}}.json"
 # has no hardware targets, where the run sent the user after a list that
 # exists nowhere.
 TARGET_PLATFORMS = frozenset({
-    "batocera", "emudeck", "knulli", "lakka", "recalbox", "retroarch", "retrobat",
+    "amberelec", "batocera", "emudeck", "knulli", "lakka", "recalbox", "retroarch", "retrobat",
     "retrodeck", "retropie", "rocknix", "romm",
 })
 RAW_FILE_URL = f"{BASE_URL}/{{path}}"
@@ -1472,7 +1472,10 @@ def _parse_bash_var(path: Path, key: str) -> str | None:
 # /usr/share/batocera to /usr/share/knulli (knulli-linux 57afa662,
 # board/scripts/post-build-script.sh:264-278) and writes KNULLI_SYSTEM_ARCH
 # to knulli.arch (package/system/knulli-system/knulli-system.mk:98-99).
+# AmberELEC's scripts/image writes COREELEC_DEVICE (a21975bf, line 168), the
+# DEVICE its Makefile builds (RG351P).
 _BOARD_SOURCES: dict[str, tuple[str, str]] = {
+    "amberelec": ("os_release", "COREELEC_DEVICE"),
     "batocera": ("file", "/usr/share/batocera/batocera.arch"),
     "recalbox": ("file", "/recalbox/recalbox.arch"),
     "rocknix": ("os_release", "HW_DEVICE"),