greybus: bootrom: Use "s2l" instead of boot stage in package name

The S3 firmware and S3-BFU firmware images will be named like this going
forward:

S3: ara_00000126_00001001_fffe0001_ffe70018_s3f.tftf
S3-BFU: ara_00000126_00001001_fffe0001_ffe70018_s3_bfu.tftf

But the current naming for S2 loader image is:

ara_00000126_00001001_fffe0001_ffe70018_02.tftf

It makes sense to use similar encoding for all three packages and so it
should be named as:

ara_00000126_00001001_fffe0001_ffe70018_s2l.tftf

Because the boot stage is passed from ES3 bootrom, we can't change its
value now. But the string created to match the package name is created
in bootrom.c and that is the only string we create from bootrom.c.

Update bootrom.c to use "s2l" instead of "02" in the package name.

Compile Tested only.

Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>
Tested-by: Karthik Ravi Shankar <karthikrs@google.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>