kernel-install(uki): filter comments from cmdline

This change aligns the behaviour of UKI generation with the behaviour
of BLS. The latter filters out lines starting with a #, allowing users
to add comments and/or temporarily remove some flags from the kernel
command line.

The kernel-install test have been adjusted to use a multiline cmdline
with a comment in it. Without this patch, the test fails.

(cherry picked from commit 0c2747e789)
(cherry picked from commit 3c5046c43f)

man/kernel-install.xml

@@ -616,9 +616,9 @@
         <term><filename>/proc/cmdline</filename></term>
         <listitem>
           <para>Specifies the kernel command line to use. The first of the files that is found will be used.
-          When running in a container, <filename>/proc/cmdline</filename> is ignored.
-          <varname>$KERNEL_INSTALL_CONF_ROOT</varname> may be used to override the search path; see below for
-          details.</para>
+          Lines starting with the <literal>#</literal> character are ignored. When running in a container,
+          <filename>/proc/cmdline</filename> is ignored. <varname>$KERNEL_INSTALL_CONF_ROOT</varname> may be
+          used to override the search path; see below for details.</para>
 
           <xi:include href="version-info.xml" xpointer="v198"/>
         </listitem>

src/kernel-install/60-ukify.install.in

@@ -185,7 +185,12 @@ def devicetree_file_location(opts) -> Optional[Path]:
 def kernel_cmdline_base() -> list[str]:
     path = input_file_location('cmdline')
     if path:
-        return path.read_text().split()
+        # Filter out commented out lines from cmdline.
+        lines = path.read_text().splitlines()
+        return [opt
+                for line in lines
+                if not line.startswith('#')
+                for opt in line.split()]
 
     # If we read /proc/cmdline, we need to do some additional filtering.
     options = Path('/proc/cmdline').read_text().split()

src/kernel-install/test-kernel-install.sh

@@ -29,7 +29,12 @@ mkdir -p "$D/sources"
 echo 'buzy image' >"$D/sources/linux"
 echo 'the initrd' >"$D/sources/initrd"
 echo 'the-token' >"$D/sources/entry-token"
-echo 'opt1 opt2' >"$D/sources/cmdline"
+
+cat >"$D/sources/cmdline" <<EOF
+opt1
+# commented line
+opt2
+EOF
 
 cat >"$D/sources/install.conf" <<EOF
 layout=bls