doc/4.bsp: Add manual for enable UART on RaspberryPi
doc/4.bsp: Add manual for enable UART on RaspberryPi

There are 2 ways to enable UART:
- Add 'ENABLE_UART = "1"' to conf/local.conf, then build the image.
or
- Update file cmdline.txt and config.txt manually in the output image.

Signed-off-by: Trung Do <64081e6b7eb2b24289b8a4e51cef9836ddf28af2@toshiba-tsdv.com>
Signed-off-by: Kazuhiro Hayashi <d6ab13038406a238a56011921c4eda80a48396fd@toshiba.co.jp>

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