fix: place standalone files in the linked folder

standalone_path is relative to the directories a standalone build
resolves it against, now declared per profile (standalone_directories).
A launcher record that links or passes a whole folder of the BIOS tree
under one of them gives that folder, and platform_layouts rebases the
standalone view once for the builder, the manifest, verify and slots
(launchers.standalone_root, common.standalone_destination).

Flycast on RetroPie and ROCKNIX reads bios/dc, on EmuDeck
bios/flycast/bios, on Batocera (FLYCAST_BIOS_PATH) and RetroBat
(Dreamcast.BiosPath) bios/dc. The ROCKNIX, Batocera and RetroBat
scrapers now record the patched data directory, the environment
variable and the setting that say so.

platforms/batocera.yml

@@ -6111,6 +6111,14 @@ launchers:
   fails_when_missing: false
   depends_on_option: gsplus_bios_filename
   source_ref: "package/batocera/core/batocera-configgen/configgen/configgen/generators/gsplus/gsplusGenerator.py:105"
+- emulator: flycast
+  build: standalone
+  kind: passes_path
+  names: []
+  search_dir: /userdata/bios/dc
+  passes_to: environment FLYCAST_BIOS_PATH
+  fails_when_missing: false
+  source_ref: "package/batocera/core/batocera-configgen/configgen/configgen/generators/flycast/flycastGenerator.py:199"
 - emulator: o2em
   build: libretro
   kind: core_option

platforms/retrobat.yml

@@ -3238,6 +3238,14 @@ launchers:
   fails_when_missing: not checked by the launcher
   depends_on_option: duck_bios
   source_ref: "emulatorLauncher/Generators/Duckstation.Generator.cs:204-209, .emulationstation/es_features.cfg:11638-11646"
+- emulator: flycast
+  build: standalone
+  kind: passes_path
+  names: []
+  search_dir: bios/dc
+  passes_to: "emu.cfg [config] Dreamcast.BiosPath = bios/dc"
+  fails_when_missing: false
+  source_ref: "emulatorLauncher/Generators/Flycast.Generator.cs:149, 167"
 - emulator: jgenesis
   build: standalone
   kind: fixed_name

platforms/rocknix.yml

@@ -757,6 +757,15 @@ launchers:
   source_ref: "projects/ROCKNIX/packages/emulators/standalone/armsx2-sa/config/S922X/ARMSX2/inis/PCSX2.ini:21,\
     \ 475, projects/ROCKNIX/packages/emulators/standalone/armsx2-sa/config/inputplumber/ARMSX2/inis/PCSX2.ini:21,\
     \ 473, projects/ROCKNIX/packages/emulators/standalone/armsx2-sa/package.mk:117-118"
+- emulator: flycast
+  build: standalone
+  kind: passes_path
+  names: []
+  search_dir: /storage/roms/dreamcast/data
+  passes_to: "find_user_data_dir() in core/linux-dist/main.cpp returns it"
+  fails_when_missing: false
+  source_ref: "projects/ROCKNIX/packages/emulators/standalone/flycast-sa/patches/000-conf-dir.patch:25,\
+    \ 29"
 - emulator: drastic
   build: standalone
   kind: links