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.
diff --git a/README b/README
index d18df54..c0a9ea0 100644
--- a/README
+++ b/README
@@ -5450,10 +5450,10 @@
 		  unset, then it will be made silent if the U-Boot console
 		  is silent.
 
-  tftpsrcport	- If this is set, the value is used for TFTP's
+  tftpsrcp	- If this is set, the value is used for TFTP's
 		  UDP source port.
 
-  tftpdstport	- If this is set, the value is used for TFTP's UDP
+  tftpdstp	- If this is set, the value is used for TFTP's UDP
 		  destination port instead of the Well Know Port 69.
 
   tftpblocksize - Block size to use for TFTP transfers; if not set,
@@ -5467,6 +5467,14 @@
 		  faster in networks with high packet loss rates or
 		  with unreliable TFTP servers.
 
+  tftptimeoutcountmax	- maximum count of TFTP timeouts (no
+		  unit, minimum value = 0). Defines how many timeouts
+		  can happen during a single file transfer before that
+		  transfer is aborted. The default is 10, and 0 means
+		  'no timeouts allowed'. Increasing this value may help
+		  downloads succeed with high packet loss rates, or with
+		  unreliable TFTP servers or client hardware.
+
   vlan		- When set to a value < 4095 the traffic over
 		  Ethernet is encapsulated/received over 802.1q
 		  VLAN tagged frames.