Makefile: shell fixes

[ydirson]

"exit -1" has traditionally had "exit 255" result, but starts to be rejected by some shells (eg. dash, which is is the default on many Debian systems):

host$ bash $ exit -1 exit host$ echo $? 255

host$ dash $ exit -1 dash: 1: exit: Illegal number: -1 $ host$ echo $? 2

Signed-off-by: Yann Dirson yann@blade-group.com

[jforissier]

Reviewed-by: Jerome Forissier <jerome@forissier.org>

[jenswi-linaro]

Reviewed-by: Jens Wiklander <jens.wiklander@linaro.org>

[jforissier]

@ydirson can you please add the review tags? Thanks.