scripts/Makefile.lib: EFI: Use capsule CRT instead of ESL file

The EFI Capsule ESL file (EFI Signature List File) used for authentication
is a binary generated from the EFI Capsule public key certificate. Instead
of including it in the source repo, automatically generate it from the
certificate file during the build process.

Currently, sandbox is the only device using this, so removed its ESL file
and set the (new) CONFIG_EFI_CAPSULE_CRT_FILE config to point to its public
key certificate.

Signed-off-by: Jonathan Humphreys <j-humphreys@ti.com>

doc/develop/uefi/uefi.rst

@@ -580,10 +580,10 @@ and used by the steps highlighted below.
     }
 
 You can perform step-4 through the Kconfig symbol
-CONFIG_EFI_CAPSULE_ESL_FILE. This symbol points to the esl file
-generated in step-2. Once the symbol has been populated with the path
-to the esl file, it will automatically get embedded into the
-platform's dtb as part of U-Boot build.
+CONFIG_EFI_CAPSULE_CRT_FILE. This symbol points to the signing key
+generated in step-2. As part of U-Boot build, the ESL certificate file will
+be generated from the signing key and automatically get embedded into the
+platform's dtb.
 
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