Adds a new config variable "GRUB_BTRFS_ROOTFLAGS" (#133)

Script:

Improves the rootflags option in the kernel command line:
- Remove "rw" flag.
- Parse fstab mountoptions and add them to rootflags:
Adds functionality to read in  snapshot specific mount flags
from the respective fstab and uses them in combination with the new 
config option, while the latter is prioritized.
Both places can, but don't need to define mount flags. If either of them
doesn't, it will simply not be taken into account.

Config file:

Adds a new config variable "GRUB_BTRFS_ROOTFLAGS":
- The new variable is used to include user-defined rootflags in the 
kernel command line of generated snapshot bootmenu entries.
See config file for more infos.

41_snapshots-btrfs

@@ -115,6 +115,13 @@ else
     LINUX_ROOT_DEVICE=UUID=${root_uuid}
 fi
 }
+## Detect rootflags
+detect_rootflags()
+{
+    local fstabflags=$(grep -oE '^\s*[^#][[:graph:]]+\s+/\s+btrfs\s+[[:graph:]]+' "${gbgmp}/${snap_dir_name}/etc/fstab" \
+                        | sed -E 's/^.*[[:space:]]([[:graph:]]+)$/\1/;s/,?subvol(id)?=[^,$]+//g;s/^,//')
+    rootflags="rootflags=${fstabflags:+$fstabflags,}${GRUB_BTRFS_ROOTFLAGS:+$GRUB_BTRFS_ROOTFLAGS,}"
+}
 
 ### Error Handling
 print_error()
@@ -183,7 +190,7 @@ make_menu_entries()
         fi
         echo 'Loading Snapshot: "${snap_date_time}" "${snap_dir_name}"'
         echo 'Loading Kernel: "${k}" ...'
-        linux \"${boot_dir_root_grub}/"${k}"\" root="${LINUX_ROOT_DEVICE}" rw ${kernel_parameters} rootflags=subvol=\""${snap_dir_name}"\""
+        linux \"${boot_dir_root_grub}/"${k}"\" root="${LINUX_ROOT_DEVICE}" ${kernel_parameters} ${rootflags}subvol=\""${snap_dir_name}"\""
                 if [[ "${name_microcode}" != "x" ]] ; then
                     entry "\
         echo 'Loading Microcode & Initramfs: "${u}" "${i}" ...'
@@ -417,6 +424,8 @@ boot_bounded()
         detect_kernel
         if [ -z "${list_kernel}" ]; then continue; fi
         name_kernel=("${list_kernel[@]##*"/"}")
+        # Detect rootflags
+        detect_rootflags
         # Initramfs (Original + custom initramfs)
         detect_initramfs
         if [ -z "${list_initramfs}" ]; then continue; fi
@@ -472,6 +481,8 @@ boot_separate()
         snap_dir_name="$(trim "$snap_dir_name")"
         snap_date_time="$(echo "$item" | cut -d' ' -f1-2)"
         snap_date_time="$(trim "$snap_date_time")"
+        # Detect rootflags
+        detect_rootflags
         # show snapshot found during run "grub-mkconfig"
         if [[ "${GRUB_BTRFS_SHOW_SNAPSHOTS_FOUND:-"true"}" = "true" ]]; then
             printf "Found snapshot: %s\n" "$item" >&2 ;

config

@@ -54,6 +54,14 @@
 # Default: ("")
 #GRUB_BTRFS_CUSTOM_MICROCODE=("custom-ucode.img" "custom-uc.img "custom_ucode.cpio")
 
+# Comma seperated mount options to be used when booting a snapshot.
+# They can be defined here as well as in the "/" line inside the respective snapshots'
+# "/etc/fstab" files.  Mount options found in both places are combined, and this variable
+# takes priority over `fstab` entries.
+# NB: Do NOT include "subvol=..." or "subvolid=..." here.
+# Default: ""
+#GRUB_BTRFS_ROOTFLAGS="space_cache,commit=10,norecovery"
+
 # Ignore specific path during run "grub-mkconfig".
 # Only exact paths are ignored.
 # e.g : if `specific path` = @, only `@` snapshot will be ignored.