commit 0edf8b5b breaks
building on a different directory with the O= parameter.
The patch wil fix this issue, generating always asm-offsets.h before
the other targets.

Signed-off-by: Stefano Babic <sbabic@denx.de>
CC: Matthias Weisser <weisserm@arcor.de>
CC: Wolfgang Denk <wd@denx.de>