build: define CPU only when arch/${ARCH}/cpu/${CPU} exists

The directory arch/${ARCH}/cpu/${CPU} does not exist
in avr32, blackfin, microblaze, nios2, openrisc, sandbox, x86.

These architectures have only one CPU type.
Defining CPU should not be required for such architectures.

This commit allows cpu field (= the 3rd field of boards.cfg)
to be kept blank.

Signed-off-by: Masahiro Yamada <yamada.m@jp.panasonic.com>
Cc: Andreas Bießmann <andreas.devel@googlemail.com>
Cc: Simon Glass <sjg@chromium.org>
Cc: Sonic Zhang <sonic.zhang@analog.com>
Cc: Michal Simek <michal.simek@xilinx.com>
Cc: Thomas Chou <thomas@wytron.com.tw>
Cc: Stefan Kristiansson <stefan.kristiansson@saunalahti.fi>