Add phony target to run pre-build hook script

Arduino.mk

@@ -465,6 +465,10 @@ else
     $(call show_config_variable,USER_LIB_PATH,[USER])
 endif
 
+ifndef PRE_BUILD_HOOK
+    PRE_BUILD_HOOK = pre-build-hook.sh
+endif
+
 ########################################################################
 # boards.txt parsing
 
@@ -1102,9 +1106,12 @@ all: 		$(TARGET_EEP) $(TARGET_HEX)
 # prerequisite" (e.g., put "| $(OBJDIR)" at the end of the prerequisite
 # list) to prevent remaking the target when any file in the directory
 # changes.
-$(OBJDIR):
+$(OBJDIR): pre-build
 		$(MKDIR) $(OBJDIR)
 
+pre-build:
+		$(call runscript_if_exists,$(PRE_BUILD_HOOK))
+
 $(TARGET_ELF): 	$(LOCAL_OBJS) $(CORE_LIB) $(OTHER_OBJS)
 		$(CC) $(LDFLAGS) -o $@ $(LOCAL_OBJS) $(CORE_LIB) $(OTHER_OBJS) -lc -lm
 
@@ -1114,6 +1121,7 @@ $(CORE_LIB):	$(CORE_OBJS) $(LIB_OBJS) $(USER_LIB_OBJS)
 error_on_caterina:
 		$(ERROR_ON_CATERINA)
 
+
 # Use submake so we can guarantee the reset happens
 # before the upload, even with make -j
 upload:		$(TARGET_HEX) verify_size
@@ -1240,7 +1248,7 @@ help:
 
 .PHONY: all upload raw_upload raw_eeprom error_on_caterina reset reset_stty ispload \
         clean depends size show_boards monitor disasm symbol_sizes generated_assembly \
-        generate_assembly verify_size burn_bootloader help
+        generate_assembly verify_size burn_bootloader help pre-build
 
 # added - in the beginning, so that we don't get an error if the file is not present
 -include $(DEPS)

Common.mk

@@ -4,6 +4,9 @@
 # (directory and optional filename) exists
 dir_if_exists = $(if $(wildcard $(1)$(2)),$(1))
 
+# Run a shell script if it exists.
+runscript_if_exists = $(if $(wildcard $(1)), $(shell sh $(1)))
+
 # For message printing: pad the right side of the first argument with spaces to
 # the number of bytes indicated by the second argument.
 space_pad_to = $(shell echo $(1) "                                                      " | head -c$(2))

arduino-mk-vars.md

@@ -776,6 +776,25 @@ MONITOR_CMD = minicom
 
 ----
 
+### PRE_BUILD_HOOK
+
+**Description:**
+
+Path to shell script to be executed before build. Could be used to automatically
+bump revision number for example.
+
+Defaults to `pre-build-hook.sh`
+
+**Example:**
+
+```Makefile
+PRE_BUILD_HOOK = ~/bin/bump-revision.sh
+```
+
+**Requirement:** *Optional*
+
+----
+
 ## Avrdude setting variables
 
 ### AVRDUDE