Add unb2c_test_1GbE_I-r7dbc0316d image.

boards/uniboard2c/images/images.txt

 Image name                                          | Date          | Author               | Usage
 ---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
 unb2c_test_heater-r6825a1822                        | 2021-12-10    | R vd Walle           | See, $UPE_GEAR/peripherals/util_heater.py.
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+unb2c_test_1GbE_I-r7dbc0316d.tar.gz                 | 2021-12-10    | E. Kooistra          | See, $UPE_GEAR/peripherals/tc_unb2_test_eth.py
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+Example:
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+# Create tar.gz file:
+> tar -cvzf unb2c_test_1GbE_I-r7dbc0316d.tar.gz unb2c_test_1GbE_I.rbf unb2c_test_1GbE_I.sof
+# Extract tar.gz file:
+> tar -xvzf unb2c_test_1GbE_I-r7dbc0316d.tar.gz