stm32f4: undef CONFIG_CMD_CHARGEN
stm32f4: undef CONFIG_CMD_CHARGEN

This patch excludes CONFIG_CMD_CHARGEN from stm32f4 build config set
explicitly. stm32f4 has its own USB console implementation, which
chargen doesn't support yet. The affected boards are polyberry,
sweetberry and stm32f446e-eval.

BUG=none
BRANCH=all
TEST=buildall

Signed-off-by: Namyoon Woo <e8732cefb861f07768f47df788a8e8eb75efc09a@chromium.org>
Change-Id: I0f3dadd55fa2c197fff32afbe3fecb773491003e
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/ec/+/2951862
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