linux-bug: fix installation

linux-bug: fix installation

without the fix it does that:
| cp: target `.../work/bug-angstrom-linux-gnueabi/linux-bug-2.6.27.2+svnr10746-r32/image/kernel//include/asm/' is not a directory
NOTE: package linux-bug-2.6.27.2+svnr10746-r32: task do_install: Failed

Thanks to eFfeM on #oe for help:
<eFfeM> you copy to $kerneldir/include/asm but that is not guaranteed to exist, ust add an install -d (install -d is better than mkdir as it will also create inbetween dirs if needed)
[...]
<eFfeM> just add the install

Signed-off-by: Denis 'GNUtoo' Carikli <GNUtoo@no-log.org>