[bootloader] factor out GRUB EFI determinations

src/modules/bootloader/main.py

@@ -24,8 +24,6 @@ import os
 import shutil
 import subprocess
 
-import platform
-
 import libcalamares
 
 from libcalamares.utils import check_target_env_call
@@ -283,6 +281,33 @@ def install_systemd_boot(efi_directory):
     create_loader(loader_path, distribution_translated)
 
 
+def get_grub_efi_parameters():
+    """
+    Returns a 3-tuple of suitable parameters for GRUB EFI installation,
+    depending on the host machine architecture. The return is
+        - target name
+        - grub.efi name
+        - boot.efi name
+    all three are strings. May return None if there is no suitable
+    set for the current machine. May return unsuitable values if the
+    host architecture is unknown (e.g. defaults to x86_64).
+    """
+    import platform
+    efi_bitness = efi_word_size()
+    cpu_type = platform.machine()
+
+    if efi_bitness == "32":
+        # Assume all 32-bitters are legacy x86
+        return ("i386-efi", "grubia32.efi", "bootia32.efi")
+    elif efi_bitness == "64" and cpu_type == "aarch64":
+        return ("arm64-efi", "grubaa64.efi", "bootaa64.efi")
+    elif efi_bitness == "64":
+        # If it's not ARM, must by AMD64
+        return ("x86_64-efi", "grubx64.efi", "bootx64.efi")
+    libcalamares.utils.warning("Could not find GRUB parameters for bits {b} and cpu {c}".format(b=repr(efi_bitness), c=repr(cpu_type)))
+    return None
+
+
 def install_grub(efi_directory, fw_type):
     """
     Installs grub as bootloader, either in pc or efi mode.
@@ -299,23 +324,8 @@ def install_grub(efi_directory, fw_type):
             os.makedirs(install_efi_directory)
 
         efi_bootloader_id = efi_label()
-        efi_bitness = efi_word_size()
 
-        cpu_type = platform.machine()
-
-        if efi_bitness == "32":
-            efi_target = "i386-efi"
-            efi_grub_file = "grubia32.efi"
-            efi_boot_file = "bootia32.efi"
-        elif efi_bitness == "64":
-            if cpu_type == "aarch64":
-                efi_target = "arm64-efi"
-                efi_grub_file = "grubaa64.efi"
-                efi_boot_file = "bootaa64.efi"
-            else:
-                efi_target = "x86_64-efi"
-                efi_grub_file = "grubx64.efi"
-                efi_boot_file = "bootx64.efi"
+        efi_target, efi_grub_file, efi_boot_file = get_grub_efi_parameters()
 
         check_target_env_call([libcalamares.job.configuration["grubInstall"],
                                "--target=" + efi_target,