riotboot: define if building the bootloader

bootloaders/riotboot/Makefile

@@ -7,6 +7,9 @@ BOARD ?= samr21-xpro
 # Select the boards with riotboot feature
 FEATURES_REQUIRED += riotboot
 
+# Provide a define to detect if building the bootloader
+CFLAGS += -DRIOTBOOT
+
 # Disable unused modules
 CFLAGS += -DNDEBUG -DLOG_LEVEL=LOG_NONE
 DISABLE_MODULE += auto_init

bootloaders/riotboot/README.md

@@ -60,6 +60,12 @@ A board capable to use riotboot must meet the following requirements:
 The above requirements are usually met if the board succeeds to execute
 the riotboot test in tests/.
 
+When building the bootloader, the global define `RIOTBOOT` is available. You
+can use this define to skip certain parts in `board_init()` (or `cpu_init()`)
+that should not be executed during boot. Note that this define is different
+from `MODULE_RIOTBOOT`, which is also defined when building an application that
+utilizes riotboot.
+
 # Single Slot
 Just compile your application with `FEATURES_REQUIRED += riotboot`. The header
 is generated automatically according to your `APP_VER`, which can be optionally