Makefile: add CFLAG -fdata-sections

This CFLAG tells the compiler to put every data bock into a own section
so gc-sections is able to remove the unused blocks.

Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>
dtls: initialize the hmac store on contiki

The call of the init function was missing. This is needed for contiki
builds.

Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>
dtls: only reserve 2 HMAC stores

For a handshake ewe only need 2 hmac stores at the same time, do not
reserve space for 3 of them.

Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>