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
1 file changed