[posix-app] add HOST flag for cross compiling (#3991)
[posix-app] add HOST flag for cross compiling (#3991)

This commit add 'HOST' to provide the option for changing compile
target host for OpenWRT.
jwhui/openthread,chshu/openthread,chshu/openthread,chshu/openthread,jwhui/openthread,bukepo/openthread,abtink/openthread,openthread/openthread,lanyuwen/openthread,librasungirl/openthread,lanyuwen/openthread,lanyuwen/openthread,openthread/openthread,librasungirl/openthread,abtink/openthread,librasungirl/openthread,bukepo/openthread,lanyuwen/openthread,lanyuwen/openthread,chshu/openthread,turon/openthread,srickardti/openthread,srickardti/openthread,bukepo/openthread,turon/openthread,openthread/openthread,srickardti/openthread,abtink/openthread,lanyuwen/openthread,jwhui/openthread,turon/openthread,abtink/openthread,bukepo/openthread,jwhui/openthread,openthread/openthread,srickardti/openthread,chshu/openthread,chshu/openthread,turon/openthread,librasungirl/openthread