Commit ea4ec3aa ("TBB: use SHA256 to generate the certificate
signatures") updated the cert_create tool to generate the
signatures using SHA256 instead of SHA1. Therefore, SHA1 is no
longer required.

This patch removes the SHA1 option from the PolarSSL configuration
file. The source file sha1.c is no longer needed and has been
excluded from the build. The SHA1 functions are no longer included
in the binary, reducing the memory footprint of BL1 and BL2 by
approximately 6 KB.

Change-Id: I72ea2cff03c0964c3eaadce148ec2ad2c6dde2e3