Add .esimximg and .esimximg.dek targets for signed and encrypted
images and their corresponding DEKs. Also add rule to generate final
.img.dek files.

As an example, adding encrypted images for imx6ull_evk would look like
this:

  FILE_barebox-nxp-imx6ull-evk-encrypted.img = start_nxp_imx6ull_evk.pblx.esimximg
  image-$(CONFIG_MACH_NXP_IMX6ULL_EVK) += barebox-nxp-imx6ull-evk-encrypted.img

  FILE_barebox-nxp-imx6ull-evk-encrypted.img.dek = start_nxp_imx6ull_evk.pblx.esimximg.dek
  image-$(CONFIG_MACH_NXP_IMX6ULL_EVK) += barebox-nxp-imx6ull-evk-encrypted.img.dek

Signed-off-by: Marcin Niestroj <m.niestroj@grinn-global.com>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>