Add -r option to mimic functionality of linux dmesg.
It will prefix log level and timestamp to each buffer:
<6>[       460us] barebox 2019.02.0-00266-g6aea757067-dirty #355 Thu Feb 21 11:51:43 CET 2019
<6>[      6279us] Board: DPTechnics DPT-Module
<6>[    209281us] mdio_bus: miibus0: probed
<6>[    210184us] ag71xx-gmac 18070000.mac@19000000.of: network device registered
<6>[    216051us] m25p80 w25q128@00: w25q128 (16384 Kbytes)
<6>[    219913us] netconsole: registered as netconsole-1
<6>[    223312us] malloc space: 0x80c00000 -> 0x80ffffff (size 4 MiB)
<6>[    228255us] eth0: got preset MAC address: c4:93:00:00:ae:89
<6>[    246363us] running /env/bin/init...

Signed-off-by: Oleksij Rempel <o.rempel@pengutronix.de>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>