execline: fix hash "sha" type spelling

sah256 -> sha256

Fixes:
http://autobuild.buildroot.net/results/9d0/9d054a26d7833353efbe49c984a7cb66d77b1d0c/



Signed-off-by: Matthew Weber <matthew.weber@rockwellcollins.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>