scripts/Makefile.lib: print error when no public key is specified

The current build system embeds the EFI Signature List(ESL)
into the dtb to be used in the EFI capsule authentication.
This ESL file is specified through the CONFIG_EFI_CAPSULE_ESL_FILE
Kconfig option. If CONFIG_EFI_CAPSULE_ESL_FILE is not specified,
U-boot build ends up with failure but the cause of failure is not
easily understandable. Current error message is as follows.

FATAL ERROR: Error reading file into data: Is a directoryCheck /home/ubuntu/src/ledge/u-boot/arch/arm/dts/.synquacer-sc2a11-developerbox.dtb.pre.tmp for errors
make[2]: *** [scripts/Makefile.lib:355: arch/arm/dts/synquacer-sc2a11-developerbox.dtb] Error 1
make[1]: *** [dts/Makefile:44: arch-dtbs] Error 2
make: *** [Makefile:1165: dts/dt.dtb] Error 2
make: *** Waiting for unfinished jobs....

This commit shows the error message that CONFIG_EFI_CAPSULE_ESL_FILE
must be specified when the EFI capsule authentication is enabled, then
terminate the build with error.

Signed-off-by: Masahisa Kojima <masahisa.kojima@linaro.org>
Reviewed-by: Weizhao Ouyang <o451686892@gmail.com>