mx8m: csf.sh: use vars for keys to avoid file edits when signing

The csf_spl.txt and csf_fit.txt templates contain file paths which must
be edited for the location of your NXP CST generated key files.

Streamline the process of signing an image by assigning unique var names
to these which can be expended from env variables in the csf.sh script.

The following vars are used:
 SRK_TABLE - full path to SRK_1_2_3_4_table.bin
 CSF_KEY - full path to the CSF Key CSF1_1_sha256_4096_65537_v3_usr_crt.pem
 IMG_KEY - full path to the IMG Key IMG1_1_sha256_4096_65537_v3_usr_crt.pem

Additionally provide an example of running the csf.sh script.

Signed-off-by: Tim Harvey <tharvey@gateworks.com>
Reviewed-by: Fabio Estevam <festevam@denx.de>
Reviewed-by: Peng Fan <peng.fan@nxp.com>