net: Fix download command parsing

net: Fix download command parsing

When CONFIG_SYS_HUSH_PARSER is defined network download
commands with 1 argument in the format 'tftp "/path/file"'
do not work as expected. The hush command parser strips
the quotes from "/path/file" which causes the network
commands to interpret "/path/file" as an address
instead of the intended filename.

The previous check for a leading quote in netboot_common()
was replaced with a check which ensures only valid
numbers are treated as addresses.

Signed-off-by: Peter Tyser <ptyser@xes-inc.com>
Signed-off-by: Ben Warren <biggerbadderben@gmail.com>