Tools: upgrade to gcc 4.8
usb_conf.h: do not redefine __packed if already defined

gcc 4.8 now provides this definition in some of its files. Keeping
the definition for other compilers though.
Tools: use the OS specific platforms to define ARM_SDK_URL

Windows now defines what to get but will still not install it
for you.
Merge pull request #1082 from peabody124/gcc48

Upgrade to gcc 4.8