Delete trailing whitespace

The following command is used to accomplish this:
    git ls-files | xargs sed -i '' -e's/[[:space:]]*$//'
the sed here is BSD version, but it is supposed to work well with
GNU version.
Merge pull request #13 from xuchunyang/patch-2

Delete trailing whitespace
Makefile: build bootloader
Makefile: build kernel

All the middle objects files are exactly under the same path as source
files, instead of move they to ./bin

Beside, since 'tool/main.ld' is changed, 'Rakefile' should
not work as we excepte.