enable CONFIG_CMD_CHARGEN
enable CONFIG_CMD_CHARGEN

This patch enables the uart command, chargen by default. It generates
a series of alphanumeric chractecters, and is useful in testing UART
related modules with a stress workload.

This patch occupies 400 bytes or so in flash memory of RW image.

BUG=b:158477297
BRANCH=all
TEST=ran it on Atlas, Puff, and Volteer.

Signed-off-by: Namyoon Woo <e8732cefb861f07768f47df788a8e8eb75efc09a@chromium.org>
Cq-Depend: chromium:1679710,chromium:1554198,chromium:2080933,chromium:2217112
Change-Id: I3c0407e7a4a6d3bb8d998e3e5618be5769192a8c
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/ec/+/2951863
Reviewed-by: Craig Hesling <383db090fd8ef907a606ac8a9eccd2962bc489cf@chromium.org>
Reviewed-by: Aseda Aboagye <12c9b286316a940fd31c24070da5ab64cdd4d0e7@chromium.org>

coreboot/chrome-ec,coreboot/chrome-ec,coreboot/chrome-ec,coreboot/chrome-ec,coreboot/chrome-ec,coreboot/chrome-ec