ARM: bootscript: Replace tftpboot with dhcp when downloading kernel

The 'dhcp' command is more versatile as it allows working with the DHCP
server to obtain serverip, ipaddress and other network parameters.

The configuration necessary to obtain the serverip (dhcpd.conf):
option option-150 code 150 = ip-address;

and in the subnet definition:
option option-150                     192.168.X.Y;

Signed-off-by: Lukasz Majewski <lukma@denx.de>