tools/genboardscfg.py: Do not output SPLCPU field

Prior to Kconfig, the CPU field of boards.cfg could optionally have
":SPLCPU", like "armv7:arm720t".
(Actually this syntax was only used for Tegra platform.)

Now it is not necessary at all because CPU is defined by
CONFIG_SYS_CPU in Kconfig.

For Tegra platform, the Kconfig option is described as follows:

  config SYS_CPU
          string
          default "arm720t" if SPL_BUILD
          default "armv7" if !SPL_BUILD

Signed-off-by: Masahiro Yamada <yamada.m@jp.panasonic.com>