hwconfig: Fix stop characters parsing for subkeys

For the following hwconfig string:

  key1:subkey1=value1,subkey2=value2;key2:value3

The subkey2 cannot be extracted correctly. The parsing code looks
for comma as a stopch, but there may be two kind of stop characters:
a comma and a semicolon.

Currently the code would return "value2;key2:value3", while just
"value2" is the correct answer.

This patch fixes the issue by making the code aware of multiple
stop characters.

For old U-Boots, the issue can be workarounded by placing a comma
before a semicolon, i.e.:

  hwconfig=key1:subkey1=value1,subkey2=value2,;key2:value3

Reported-by: York Sun <yorksun@freescale.com>
Signed-off-by: Anton Vorontsov <avorontsov@mvista.com>