Unfortunately there was root_name and root_derivation confusion
in the past for classes derived from BIP_32_HD_Wallet.
Address derivation used root_name and so would begin with 'x/'
whereas it should have begun with root_derivation, and so started
with 'm/'.
This fixes that old wart and removes some fudges from the trezor
code that used to work around it.
3d781a2d1b