docs: define __DOXYGEN__ for doxygen builds

Some docs depended on __DOXYGEN__ but this was never defined anywhere.

components/app_update/include/esp_ota_ops.h

@@ -326,9 +326,9 @@ typedef enum {
 /**
  * @brief Revokes the old signature digest. To be called in the application after the rollback logic.
  *
- * Relevant for Secure boot v2 on ESP32-S2 where upto 3 key digests can be stored (Key #N-1, Key #N, Key #N+1).
- * When key #N-1 used to sign an app is invalidated, an OTA update is to be sent with an app signed with key #N-1 & Key #N.
- * After successfully booting the OTA app should call this function to revoke Key #N-1.
+ * Relevant for Secure boot v2 on targets where upto 3 key digests can be stored (Key N-1, Key N, Key N+1).
+ * When key N-1 used to sign an app is invalidated, an OTA update is to be sent with an app signed with key N-1 & Key N.
+ * After successfully booting the OTA app should call this function to revoke Key N-1.
  *
  * @param index - The index of the signature block to be revoked
  *

docs/doxygen/Doxyfile_common

@@ -224,6 +224,7 @@ MACRO_EXPANSION        = YES
 EXPAND_ONLY_PREDEF     = YES
 PREDEFINED             = \
     $(ENV_DOXYGEN_DEFINES) \
+    __DOXYGEN__=1 \
     __attribute__(x)= \
 	_Static_assert()= \
     IDF_DEPRECATED(X)= \