Finish converting CONFIG_BOOTM_NETBSD et al to Kconfig

Finish converting CONFIG_BOOTM_NETBSD et al to Kconfig

This converts the following to Kconfig:
   CONFIG_BOOTM_NETBSD
   CONFIG_BOOTM_RTEMS
   CONFIG_DESIGNWARE_WATCHDOG
   CONFIG_DISPLAY_CPUINFO
   CONFIG_DM_ETH
   CONFIG_DM_MMC
   CONFIG_DM_REGULATOR
   CONFIG_DM_SPI
   CONFIG_DM_SPI_FLASH
   CONFIG_ISO_PARTITION
   CONFIG_OF_SEPARATE
   CONFIG_SPI_FLASH_WINBOND
   CONFIG_SPL_ETH
   CONFIG_TIMER
   CONFIG_USB_DWC3
   CONFIG_USB_DWC3_GADGET
   CONFIG_USB_DWC3_OMAP
   CONFIG_USB_DWC3_PHY_OMAP
   CONFIG_USB_EHCI_TEGRA
   CONFIG_USB_GADGET_DOWNLOAD
   CONFIG_USB_GADGET_DUALSPEED
   CONFIG_USB_GADGET_MANUFACTURER
   CONFIG_USB_GADGET_PRODUCT_NUM
   CONFIG_USB_GADGET_VBUS_DRAW
   CONFIG_USB_GADGET_VENDOR_NUM

This catches a number of cases where board config files were #undef
various CONFIG options when building SPL, and that doesn't work.  Clean
up the related comments as well.

Signed-off-by: Tom Rini <trini@konsulko.com>