C2C link doesn't come up after unblockAXI after toggling firmware on KU15P

[rzouCERN]

With the current develop branch (GIT_HASH 4a14db5), the C2C link stops working after toggling the firmware on the KU15P.

To reproduce the issue:
on cms@apollo09:

1. Set up the current develop branch of BUTool

2. program both FPGAs with firmwares with up-to-date c2c configs:

svfplayer rzou/top_Cornell_rev1_p1_KU15p-2-SM_7s.svf PLXVC.XVC_1
svfplayer rzou/top_Cornell_rev1_p2_VU7p-1-SM_7s.svf PLXVC.XVC_1

3. both FPGAs should appear under status after
   unblockAXI

4. program the KU15P with an older firmware:
   svfplayer rzou/top_Cornell_rev1_p1_KU15p-2.svf PLXVC.XVC_1

5. program back the original firmware and do unblockAXI:

svfplayer rzou/top_Cornell_rev1_p1_KU15p-2-SM_7s.svf PLXVC.XVC_1
unblockAXI

KU15P disappears from status in BUTool
One can get back the link by disabling phy contrl and manually initializing the link, but there will be errors.