allow to customize how many kernels to keep on gentoo system upgrade

sau/gentoo.py

@@ -196,12 +196,16 @@ def pkg_upgrade():
                     log.warning(line)
 
     if do_grub:
+        keep_kernels = conf.getint('default', 'keep_kernels', default=4)
+        if keep_kernels < 1:
+            log.error('keep_kernels cannot be less than one; falling back to default')
+            keep_kernels = 4
         for root, dirs, files in os.walk('/boot'):
             for sysfile in ['config', 'initramfs', 'System.map', 'vmlinuz']:
                 match = sorted(
                         [f for f in files if f.startswith(f'{sysfile}-')],
                         reverse=True)
-                for f in match[4:]:
+                for f in match[keep_kernels:]:
                     log.debug(f"Removing old kernel file {f}")
                     os.remove(os.path.join(root, f))
             break