Add to SCP binary the possibility to skip host check

[ggreco]

The ssh client provided by dropbear give the user a way to accept/skip host validation check using -y option, the -y option can be specified once to get something like this (accept):

/usr/bin/dhclient: 
Host '10.0.0.237' key accepted unconditionally.

Or twice to skip the validation:

/usr/bin/dhclient: Caution, skipping hostkey check for 10.0.0.237

This PR adds the same capability to the scp command:

iMac-2 $ /tmp/dropbear/bin/scp -y release.sh root@10.0.0.237:/tmp
/tmp/dropbear/bin/dhclient: 
Host '10.0.0.237' key accepted unconditionally.
(ssh-rsa fingerprint SHA256:QKOF+XXXXXXXXXXXXX)
root@10.0.0.237's password: 

or

iMac-2 $ /tmp/dropbear/bin/scp -y -y release.sh root@10.0.0.237:/tmp
/tmp/dropbear/bin/dhclient: Caution, skipping hostkey check for 10.0.0.237
root@10.0.0.237's password: 

[mkj]

scp -o stricthostkeychecking should work since #269