Export flash + option-byte internals to apps

Enable all furi_hal_flash_* functions: geometry, free-page info, raw
program/erase/dword-write, and option-byte read/set/apply. Lets an app
read and modify internal flash directly (research use). Guarded by
EXPORT_FLASH (default 1); build with EXPORT_FLASH=0 to leave disabled.
17 symbols exported; firmware links and the app builds.

README.md

@@ -182,6 +182,11 @@ symbol costs one API-table entry.
   temperature, remaining/full/design capacity, health, charge state) are
   already exported by stock firmware. This build also enables
   `furi_hal_power_insomnia_level`; `furi_hal_power_init` stays disabled.
+* **Flash + option-byte internals** (`furi_hal_flash_*`): flash geometry,
+  free-page info, and raw program / erase / dword-write / option-byte access.
+  An app can read and modify internal flash directly. **This can brick the
+  device or wipe storage.** Enabled here for research; build with
+  `EXPORT_FLASH=0 ./build.sh` to leave these disabled.
 
 `build.sh` also widens the GAP roles the firmware initialises
 (`GAP_PERIPHERAL_ROLE | GAP_CENTRAL_ROLE | GAP_OBSERVER_ROLE`) in

build.sh

@@ -119,6 +119,15 @@ EXPORT_ENABLE+='
 EXPORT_ENABLE+='
 ^furi_hal_power_insomnia_level$
 '
+# Flash + option-byte internals: geometry, free-page info, and raw program /
+# erase / write / option-byte access. These let an app read and modify internal
+# flash directly. WARNING: misuse can brick the device or wipe storage; enabled
+# here for research. Set EXPORT_FLASH=0 before building to leave them disabled.
+if [ "${EXPORT_FLASH:-1}" = "1" ]; then
+EXPORT_ENABLE+='
+^furi_hal_flash_
+'
+fi
 export EXPORT_ENABLE
 if grep -q "^Function,?," "$API_CSV" || ! grep -qE "^Header,\+,.*ble_hci_le\.h" "$API_CSV"; then
     # fbt notices the new headers, rewrites the csv with '?' entries and stops.