Rename CONFIG_RNDIS_TX_BUF_* to CFG_RNDIS_TX_BUF_* and rename
CONFIG_RNDIS_CMD_BUF_* to CFG_RNDIS_CMD_BUF_*. The CONFIG options where
not exposed in Kconfig so limit use of CONFIG_ to Kconfig only symbols.
Signed-off-by: Kumar Gala <kumar.gala@linaro.org>