Add kernelbuild benchmark

with this benchmark, it is possible to compare the performance of NRE and
NUL. The needed dist/imgs/kernelbuild-e2fs.bz2 can be taken from NUL imgs
repo. On my Ivy Bridge box the results are as follows:

NUL x86_32: 485 s
NRE x86_64: 484 s
NRE x86_32: 484 s (vancouver was run with only 900M of mem, 950M failed)
Merge commit 'f52e7a3ce834961867eec639f73f6351fe37e800'
Added kernelbuild-e2fs.bz2 to dist/download.sh