tizen: bootscript: Set eth0 ip addr only for flash boot mode

tizen: bootscript: Set eth0 ip addr only for flash boot mode

The lpi-4a board has two ethernet interfaces and setting
ipaddr without an interface, it tries all interface for all
enabled autoconfiguration protocol. Sometimes, this causes
boot hang to get recv from an ethernet interface for an
autoconfiguration protocol. To remove the possible boot hang,
set only eth0 ip addr for flash boot mode.

Refer ip= part of below kernel doc:
  https://git.tizen.org/cgit/platform/kernel/linux-thead/tree/Documentation/admin-guide/nfs/nfsroot.rst?h=tizen

Change-Id: I3aa5d59551aea147cd14d464eb501fcc0e843d54
Signed-off-by: Seung-Woo Kim <sw0312.kim@samsung.com>