llsec: Added READMEs

core/net/llsec/README.md

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+Link layer security is implemented as a new netstack layer, which is located in between the MAC and the NETWORK layer. The interface of LLSEC drivers is defined in [llsec.h](llsec.h). Additionally, LLSEC drivers may define a special [FRAMER](../mac/framer.h), which calls the actual FRAMER underneath. By default, the LLSEC driver `nullsec` is used, which does nothing.
+
+# TODO
+
+* Most main files do not call LLSEC.init, yet

core/net/llsec/noncoresec/README.md

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+`noncoresec` is a noncompromise-resilient 802.15.4 security implementation, which uses a network-wide key. Add these lines to your `project_conf.h` to enable `noncoresec`:
+
+```c
+#undef LLSEC802154_CONF_ENABLED
+#define LLSEC802154_CONF_ENABLED          1
+#undef NETSTACK_CONF_FRAMER
+#define NETSTACK_CONF_FRAMER              noncoresec_framer
+#undef NETSTACK_CONF_LLSEC
+#define NETSTACK_CONF_LLSEC               noncoresec_driver
+#undef NONCORESEC_CONF_SEC_LVL
+#define NONCORESEC_CONF_SEC_LVL           1
+```
+`NONCORESEC_CONF_SEC_LVL` defines the length of MICs and whether encryption is enabled or not.
+
+Setting the network-wide key works as follows:
+```c
+#define NONCORESEC_CONF_KEY { 0x00 , 0x01 , 0x02 , 0x03 , \
+                              0x04 , 0x05 , 0x06 , 0x07 , \ 
+                              0x08 , 0x09 , 0x0A , 0x0B , \ 
+                              0x0C , 0x0D , 0x0E , 0x0F } 
+```