meson.build: Detect bzip2 program

The --enable-bzip2/--disable-bzip2 configure arguments are
somehow misleading, they check for the bzip2 library, not
the bzip2 program.

We need the bzip2 program to install the EDK2 firmware blobs
(see commit 623ef63 "configure: Check bzip2 is available").

Check if the bzip2 program in the global meson.build to avoid
the configuration to succeed, but a later when trying to install
the firmware blobs:

    ../pc-bios/meson.build:5:2: ERROR: Program 'bzip2' not found

Reported-by: John Snow <jsnow@redhat.com>
Suggested-by: Paolo Bonzini <pbonzini@redhat.com>
Fixes: c8d5450 ("configure: move install_blobs from configure to meson")
Signed-off-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Message-Id: <20210114174509.2944817-3-philmd@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

meson.build

@@ -96,6 +96,8 @@ if get_option('install_blobs')
   endforeach
 endif
 
+bzip2 = find_program('bzip2', required: install_edk2_blobs)
+
 ##################
 # Compiler flags #
 ##################

pc-bios/meson.build

@@ -1,5 +1,4 @@
 if install_edk2_blobs
-  bzip2 = find_program('bzip2', required: true)
   fds = [
     'edk2-aarch64-code.fd',
     'edk2-arm-code.fd',