net: TFTP: variables cleanup and addition

TFTP source and destination port variable names are
'tftpsrcp' and 'tftpdstp' in the code, but 'tftpsrcport'
and 'tftpdstport' in the README file. Fix the README.

Add environment variable 'tftptimeoutcountmax'. As per the
comments about the global variable tftp_timeout_count_max,
make sure tftptimeoutcountmax is nonnegative.

Introduce configuration option CONFIG_NET_TFTP_VARS,
which controls whether environment variables tftpblocksize,
tftptimeout, and tftptimoueoutcountmax are read by the TFTP
client code. CONFIG_NET_TFTP_VARS defaults to y but can be
set to n by targets with to tight size contraints.

Make bf527-ezkit set CONFIG_NET_TFTP_VARS to n to keep the
target size below limit.