Fixed cross compilation issues with configure.ac

    * configure.ac: Fixed host_alias being set. Use $host_alias instead
    of $host for cross compilation checks
    * generate.cortex-m3: Added shebang
Small fixes to make cubic and rijndael build correctly
Compiling in sbrk, jrand, etc is now conditional, selected by configure.

    * configure.ac: Added options to compile in extra platform code
    * generate.cortex-m3: Enable platform code
    * src/2dfir/Makefile.am: Conditionally compile in platform code
    * src/blowfish/Makefile.am: Likewise
    * src/crc32/Makefile.am: Likewise
    * src/cubic/Makefile.am: Likewise
    * src/dijkstra/Makefile.am: Likewise
    * src/fdct/Makefile.am: Likewise
    * src/float_matmult/Makefile.am: Likewise
    * src/int_matmult/Makefile.am: Likewise
    * src/rijndael/Makefile.am: Likewise
    * src/sha/Makefile.am: Likewise
    * src/platformcode/platformcode.h: Conditionally disable jrand
Added scripts and fixes for AVR support.

    * src/rijndael/aesxam.c: Fix to compile on AVR
    * generate.avr: Created.