NIST: increase RSA key length to 2048 bit

According to NIST 800-131A, RSA key lenght for digital signature
must >= 2048 bit. Now we use 1024 bit key to generate x509 cert
file. Need to increase the key length to 2048 bit.

Change-Id: I59f614b5d8a79f9e0a96503867cfca176be5c757
Closes-Bug: 1369487
Merge "NIST: increase RSA key length to 2048 bit"