Config.sub changes to support pic32

    * README.md: Description of changes in case they need to be
        recreated for a new config.sub
    * build-aux/config.sub: Added pic32 as a valid cross compilation
        target
Updated generate files to work out of tree

    * generate.avr: Updated to work out of tree, and accept CFLAGS
    * generate.cortex-m0: Updated to work out of tree, and accept CFLAGS
    * generate.cortex-m3: Updated to work out of tree, and accept CFLAGS
    * generate.fram: Updated to work out of tree, and accept CFLAGS
    * generate.generic: Updated to work out of tree, and accept CFLAGS
    * generate.msp430: Updated to work out of tree, and accept CFLAGS
    * generate.pic32: Updated to work out of tree, and accept CFLAGS