docs: platform: adi: Change build instructions

1. Due to update of BL2 logging system,
flag -DTFM_BL2_LOG_LEVEL=LOG_LEVEL_INFO is used
to print bl2 message
2. Add instructions of converting tfm_s_signed.bin
to tfm_s_signed.hex

Change-Id: Ic88af29ba4028ee8a9b856cddccd1589f1a849a5
Signed-off-by: Hao Zhang <Hao.Zhang@analog.com>
1 file changed