libnspr: Add dependency on !BR2_xtensa and !BR2_bfin

Xtensa is not yet supported in libnspr. Also add dependencies to libnss,
which requires libnspr, and ecryptfs-utils, which requires libnss.

Fixes (xtensa)
http://autobuild.buildroot.net/results/14b/14ba6426edbffa100de924aa69157b3f59368ff2/

Fixes (blackfin)
http://autobuild.buildroot.net/results/dd3/dd353703ed94cec48cd126d80cd2f1b039811be4/



[baruch: fix summary, add autobuild reference, add comment dependency,
 add blackfin]

Signed-off-by: Chris Zankel <chris@zankel.net>
Signed-off-by: Baruch Siach <baruch@tkos.co.il>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>