doc: Make bootloaders visible in modules

bootloaders/doc.txt

@@ -0,0 +1,5 @@
+/**
+ * @defgroup bootloaders Bootloaders
+ * @brief Applications that facilitate the startup process in presence of upgradable firmware
+ *
+ */

bootloaders/riotboot/doc.txt

@@ -1,3 +1,7 @@
+/**
+@defgroup    bootloader_riotboot riotboot
+@ingroup     bootloaders
+
 # Overview
 This folder contains a simple bootloader called "riotboot".
 A header with metadata of length `RIOTBOOT_HDR_LEN` precedes
@@ -151,3 +155,5 @@ offset and a defined firmware size. To get an idea look at
 Additional remarks:
 
 - If your are in Big-Endian, you may need to further adapt some part of the code.
+
+*/

bootloaders/riotboot/main.c

@@ -8,7 +8,6 @@
  */
 
 /**
- * @defgroup    bootloaders    RIOT compatible bootloaders
  * @ingroup     bootloaders
  * @{
  *

doc/doxygen/riot.doxyfile

@@ -757,6 +757,7 @@ INPUT                  = ../../doc.txt \
                          ../../core \
                          ../../cpu \
                          ../../boards \
+                         ../../bootloaders \
                          ../../drivers \
                          ../../pkg \
                          ../../sys \