Usually the presence of
logcat_cp_boot_radio*.log files point to an issue with the phone's modem. It's possible that when you first rooted the device you flashed a ROM which contained a modem firmware that was either old, or slightly incompatible with your current carrier. Besides amassing large amounts of these log files, you would see frequent disconnects and reconnects from cell and data services when the radio crashes and restarts.
While you can always keep deleting the log files every now and then to free up the memory, the larger issue will persist. In order to resolve the root cause of the RIL crash, you will need to install the proper radio firmware. These differ based on geographic location, and sometimes on the carrier.
The first step is to find the modem version that is currently installed on your phone. You can find this information in Settings -> About phone, under the "Baseband version." It should start with "i19100", and then followed by some letters and numbers. The letters represent a geographic location:
CE---> Belgium, Luxembourg, Netherlands
DX---> Indonesia, Malaysia, Philippines, Singapore, Vietnam
DZ---> Malaysia, Singapore
JA---> South Africa
JC---> Algeria, Morocco, Nigeria, South Africa, Tunisia
JP---> Arabic, Algeria, Egypt, Iran, Iraq, Kuwait, Morocco, Nigeria,
Oman, Pakistan, Saudi Arabia, Syria
JV---> Tunisia, Turkey
UG---> North America
UH---> Latin America, The Caribbean
XE---> Bulgaria, Estonia, Kazakhstan, Latvia, Lithuania, Russia,
XX---> Austria, Belgium, France, Germany, Hungary, United Kingdom
XW---> Austria, Belgium, France, Germany, Hungary, Italy, Nordic,
Spain, United Kingdom
ZC---> China, Hong Kong
ZH---> Hong Kong
ZS---> China, Hong Kong
And the numbers represent the version, incremented alphabetically. You should find the latest modem version based on the country you are in, and then flash it on your phone using either Odin, or Recovery (must be a custom recovery like ClockworkMod or TWRP, for example.)
A quick Google search yielded this site which contains ClockworkMod-formatted modems for the Galaxy S2 I9100 variant for various countries. I'm not sure how recent it is, but this should start you off in the right direction.