tools/imx8m_image.sh: remove bashism

Use a single '=' to test string equality for compatibility with non-bash
shells. Otherwise, if /bin/sh is dash, build fails:

./tools/imx8m_image.sh: 15: [: signed_hdmi_imx8m.bin: unexpected operator
./tools/imx8m_image.sh: 15: [: signed_hdmi_imx8m.bin: unexpected operator
./tools/imx8m_image.sh: 15: [: spl/u-boot-spl-ddr.bin: unexpected operator
./tools/imx8m_image.sh: 15: [: spl/u-boot-spl-ddr.bin: unexpected operator
WARNING './spl/u-boot-spl-ddr.bin' not found, resulting binary is not-functional

Signed-off-by: Baruch Siach <baruch@tkos.co.il>
Tested-by: Chris Spencer <christopher.spencer@sea.co.uk>