[PATCH v2] mx8m: csf.sh: use vars for keys to avoid file edits when signing

[sbabic at denx.de]

> 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 at gateworks.com>
> Reviewed-by: Fabio Estevam <festevam at denx.de>
> Reviewed-by: Peng Fan <peng.fan at nxp.com>
Applied to u-boot-imx, master, thanks !

Best regards,
Stefano Babic

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