meson: Use find_program() to resolve the entitlement.sh script

meson: Use find_program() to resolve the entitlement.sh script

Using ../configure without any particular option generates 31 targets
on Darwin, and meson search for the entitlement.sh script 31 times:

  Program nm found: YES
  Program scripts/undefsym.py found: YES (/opt/homebrew/opt/python@3.9/bin/python3.9 /Code/qemu/scripts/undefsym.py)
  Program scripts/feature_to_c.sh found: YES (/bin/sh /Code/qemu/scripts/feature_to_c.sh)
  Program scripts/entitlement.sh found: YES (/Code/qemu/scripts/entitlement.sh)
  Program scripts/entitlement.sh found: YES (/Code/qemu/scripts/entitlement.sh)
  Program scripts/entitlement.sh found: YES (/Code/qemu/scripts/entitlement.sh)
  Program scripts/entitlement.sh found: YES (/Code/qemu/scripts/entitlement.sh)
  Program scripts/entitlement.sh found: YES (/Code/qemu/scripts/entitlement.sh)
  Program scripts/entitlement.sh found: YES (/Code/qemu/scripts/entitlement.sh)
  Program scripts/entitlement.sh found: YES (/Code/qemu/scripts/entitlement.sh)
  Program scripts/entitlement.sh found: YES (/Code/qemu/scripts/entitlement.sh)
  Program scripts/entitlement.sh found: YES (/Code/qemu/scripts/entitlement.sh)
  Program scripts/entitlement.sh found: YES (/Code/qemu/scripts/entitlement.sh)
  Program scripts/entitlement.sh found: YES (/Code/qemu/scripts/entitlement.sh)
  Program scripts/entitlement.sh found: YES (/Code/qemu/scripts/entitlement.sh)
  Program scripts/entitlement.sh found: YES (/Code/qemu/scripts/entitlement.sh)
  Program scripts/entitlement.sh found: YES (/Code/qemu/scripts/entitlement.sh)
  Program scripts/entitlement.sh found: YES (/Code/qemu/scripts/entitlement.sh)
  Program scripts/entitlement.sh found: YES (/Code/qemu/scripts/entitlement.sh)
  Program scripts/entitlement.sh found: YES (/Code/qemu/scripts/entitlement.sh)
  Program scripts/entitlement.sh found: YES (/Code/qemu/scripts/entitlement.sh)
  Program scripts/entitlement.sh found: YES (/Code/qemu/scripts/entitlement.sh)
  Program scripts/entitlement.sh found: YES (/Code/qemu/scripts/entitlement.sh)
  Program scripts/entitlement.sh found: YES (/Code/qemu/scripts/entitlement.sh)
  Program scripts/entitlement.sh found: YES (/Code/qemu/scripts/entitlement.sh)
  Program scripts/entitlement.sh found: YES (/Code/qemu/scripts/entitlement.sh)
  Program scripts/entitlement.sh found: YES (/Code/qemu/scripts/entitlement.sh)
  Program scripts/entitlement.sh found: YES (/Code/qemu/scripts/entitlement.sh)
  Program scripts/entitlement.sh found: YES (/Code/qemu/scripts/entitlement.sh)
  Program scripts/entitlement.sh found: YES (/Code/qemu/scripts/entitlement.sh)
  Program scripts/entitlement.sh found: YES (/Code/qemu/scripts/entitlement.sh)
  Program scripts/entitlement.sh found: YES (/Code/qemu/scripts/entitlement.sh)
  Program scripts/entitlement.sh found: YES (/Code/qemu/scripts/entitlement.sh)
  Program scripts/entitlement.sh found: YES (/Code/qemu/scripts/entitlement.sh)
  Configuring 50-edk2-i386-secure.json using configuration
  Configuring 50-edk2-x86_64-secure.json using configuration

Use find_program() which seems to cache the script path once found.

Signed-off-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Reviewed-by: Akihiko Odaki <akihiko.odaki@gmail.com>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-Id: <20220122002052.83745-1-f4bug@amsat.org>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>