buildrpm et al on the XO-1.75

[Yioryos Asprobounitis]

I was trying few things with the arm-3.0-wip kernel and was building fine in both x86_64 machines and the XO-1.75 itself! 
However, buildrpm had some problems. For one, it defaults to /tmp as a builddir which makes it unusable in any XO. I can understand that is an infrastructure script but defaulting to maybe $HOME and cleaning up at the end, can serve all cases.

The other problem I had on the XO-1.75(os13) was that although the kernel was building fine, the rpm building failed with:
`error: create archive failed on file /home/olpc/kernel_sources/olpc-2.6/olpc/SOURCES/olpc-3.0.tar.bz2: cpio: Bad magic' 
That's too cryptic for me I'm afraid. Any pointers?
Could it be becase the source was patched? 

Regarding the kernel changes I tried, I noticed that both usb and sound fail to build as modules but they are OK in the kernel.
The problem is actually during modpost where are sound/soc/mmp/snd-soc-olpc-xo-1-75.ko and  drivers/usb/host/ehci-hcd.ko are found "undefined" and thus module building fails.
If anyone has any ideas...
Thx