`newt target amend` does not work

[andrzej-kaczmarek]

dd96417 broke newt target amend command:

andk@t480s:~/devel/mynewt$ newt target amend blehci syscfg=BLE_LL_CFG_FEAT_LE_2M_PHY=1:BLE_LL_CFG_FEAT_CODED_PHY=1
Amended syscfg for Target targets/blehci successfully
andk@t480s:~/devel/mynewt$ newt target show blehci
targets/blehci
    app=@apache-mynewt-nimble/apps/blehci
    bsp=@apache-mynewt-core/hw/bsp/fanstel-ev-bt840
    build_profile=debug
andk@t480s:~/devel/mynewt$ cat targets/blehci/syscfg.yml 
syscfg.vals: "map[BLE_LL_CFG_FEAT_LE_2M_PHY:1 BLE_LL_CFG_FEAT_CODED_PHY:1]"

Seems like new settings to write are passed to KvToYaml as map[string]string which is not the same as map[interface{}]interface{} so it is handled by default case and just converted to string representation of map. I have no idea how to fix this properly though...