zephyr.bbclass: Add ability to set board

Currently, there is no ability to set the board used by Zephyr.  This
limits the ability to set the board to something other than the machine
name.  By setting up an intermediate variable, ZEPHYR_BOARD, this can be
set by those that know better (with the default to use the machine name
as before).

Signed-off-by: Jon Mason <jon.mason@arm.com>

classes/zephyr.bbclass

@@ -1,5 +1,3 @@
-
-
 inherit terminal
 
 OE_TERMINAL_EXPORTS += "HOST_EXTRACFLAGS HOSTLDFLAGS TERMINFO CROSS_CURSES_LIB CROSS_CURSES_INC"
@@ -10,11 +8,12 @@ CROSS_CURSES_INC = '-DCURSES_LOC="<curses.h>"'
 TERMINFO = "${STAGING_DATADIR_NATIVE}/terminfo"
 
 KCONFIG_CONFIG_COMMAND ??= "menuconfig"
+ZEPHYR_BOARD ?= "${MACHINE}"
 
 python () {
     # Translate MACHINE into Zephyr BOARD
     # Zephyr BOARD is basically our MACHINE, except we must use "-" instead of "_"
-    board = d.getVar('MACHINE',True)
+    board = d.getVar('ZEPHYR_BOARD', True)
     board = board.replace('-', '_')
     d.setVar('BOARD',board)
 }