Add new *_VER variables to documentation

HISTORY.md

@@ -18,6 +18,7 @@ I tried to give credit whenever possible. If I have missed anyone, kindly add it
 - Tweak: Update Windows usage documentation and allow non-relative paths (issue #519) (https://github.com/tuna-f1sh)
 - Tweak: Support Cygwin Unix Python and Windows installation on Windows to pass correct port binding. (https://github.com/tuna-f1sh)
 - Tweak: Update how avr-size is called on Sam, also moved to gnu11 std (issue #602) (https://github.com/tuna-f1sh)
+- Tweak: Detect most recent toolchain if multiple found, add `*_VER` variable to override (issue #611) (https://github.com/tuna-f1sh)
 - New: Added -fdiagnostics-color to \*STD flags (https://github.com/sej7278)
 - New: Made -fdiagnostics-color take a variiable DIAGNOSTICS_COLOR_WHEN: never, always, auto. (https://github.com/wingunder)
 - New: Add generation of tags file using ctags, which automatically includes project libs and Arduino core. (https://github.com/tuna-f1sh)

arduino-mk-vars.md

@@ -97,6 +97,25 @@ ARM_TOOLS_DIR =
 
 ----
 
+### ARM_TOOLS_VER
+
+**Description:**
+
+Sub-directory where the arm toolchain is installed - usually the tool version.
+
+Can usually be detected from `$ARDUINO_PACKAGE_DIR` /tools subdirectory when ARM
+device support is installed. Will resolve latest version if multiple found.
+
+**Example:**
+
+```Makefile
+ARM_TOOLS_VER = 7-2017q4
+```
+
+**Requirement:** *Optional*
+
+----
+
 ### RESET_CMD
 
 **Description:**
@@ -1798,6 +1817,16 @@ device support is installed.
 
 ----
 
+### BOSSA_VER
+
+**Description:**
+
+`bossa` sub-directory - usually the tool version. Will auto-detect to highest version found.
+
+**Requirement:** *Optional*
+
+----
+
 ### BOSSA_OPTS
 
 **Description:**
@@ -1823,6 +1852,16 @@ device support is installed.
 
 ----
 
+### OPENOCD_VER
+
+**Description:**
+
+`openocd` sub-directory - usually the tool version. Will auto-detect to highest version found.
+
+**Requirement:** *Optional*
+
+----
+
 ### OPENOCD_OPTS
 
 **Description:**