Add BOOTLOADER_PARENT which has to be set if the user sets either of the other two BOOTLOADER variables

Fix #178

HISTORY.md

@@ -15,6 +15,7 @@ I tried to give credit whenever possible. If I have missed anyone, kindly add it
 - Fix: AVR tools paths for chipKIT in Linux. (https://github.com/peplin)
 - Fix: Consider usb or usb:... to be a valid ISP_PORT (https://github.com/geoffholden)
 - Add: Add phony target to run pre-build hook script (https://github.com/jrid)
+- Fix: Add BOOTLOADER_PARENT to `arduino-mk-vars.md` and fixed BOOTLOADER_PATH example. (https://github.com/sej7278)
 
 ### 1.3.1 (2014-02-04)
 - Fix: BUNDLED_AVR_TOOLS_DIR is now set properly when using only arduino-core and not the whole arduino package. (https://github.com/sej7278)

arduino-mk-vars.md

@@ -1026,7 +1026,7 @@ BOOTLOADER_FILE = optiboot_atmega328.hex
 
 **Description:**
 
-Path to bootloader file.
+Relative path to bootloader directory.
 
 Usually can be auto-detected as a relative `bootloader.path` from `boards.txt`
 
@@ -1036,14 +1036,34 @@ Usually can be auto-detected as a relative `bootloader.path` from `boards.txt`
 BOOTLOADER_PATH = optiboot
 # or
 BOOTLOADER_PATH = arduino:atmega
-# or
-BOOTLOADER_PATH = /usr/share/arduino/hardware/arduino/bootloaders/caterina/Caterina-Esplora.hex
 ```
 
 **Requirement:** *Optional*
 
 ----
 
+### BOOTLOADER_PARENT
+
+**Description:**
+
+Absolute path to bootloader file's parent directory.
+
+Defaults to `/usr/share/arduino/hardware/arduino/bootloaders` (Linux)
+
+**Example:**
+
+```Makefile
+BOOTLOADER_PARENT = ~/sketchbook/hardware/promicro/bootloaders
+BOOTLOADER_PATH  = caterina
+BOOTLOADER_FILE  = Caterina-promicro16.hex
+```
+
+Would result in an absolute path to the bootloader hex file of `~/sketchbook/hardware/promicro/bootloaders/caterina/Caterina-promicro16.hex`
+
+**Requirement:** *Optional, unless BOOTLOADER_FILE and/or BOOTLOADER_PATH are user-defined*
+
+----
+
 ## ChipKIT variables
 
 ### MPIDE_DIR