BeagleBoard: Allow to overwrite the default toolchain

Each OS has its own default EDK2 toolchain. This change allows to overwrite
this default option.
Example: On Linux the default toolchain is ARMLINUXGCC, this command line can overwrite it:
TARGET_TOOLS=ARMGCC ./build.sh



git-svn-id: https://edk2.svn.sourceforge.net/svnroot/edk2/trunk/edk2@12901 6f19259b-4bc3-4df7-8a09-765794883524

BeagleBoardPkg/build.sh

@@ -51,8 +51,10 @@ else
 fi
 
 #
-# Pick a default tool type for a given OS
+# Pick a default tool type for a given OS if no toolchain already defined
 #
+if [ -z "$TARGET_TOOLS" ]
+then
   case `uname` in
     CYGWIN*) 
       TARGET_TOOLS=RVCT31CYGWIN 
@@ -75,6 +77,7 @@ case `uname` in
       fi  
       ;;
   esac
+fi
 
 TARGET=DEBUG
 for arg in "$@"