cmd: gpt: Add gpt_partition_bootable variable

Adds an additional variable called gpt_partition_bootable that indicates
if the given partition is bootable or not.

Signed-off-by: Joshua Watt <JPEWhacker@gmail.com>
diff --git a/doc/usage/cmd/gpt.rst b/doc/usage/cmd/gpt.rst
index 6387c81..b505b15 100644
--- a/doc/usage/cmd/gpt.rst
+++ b/doc/usage/cmd/gpt.rst
@@ -108,6 +108,9 @@
 gpt_partition_entry
     the partition number in the table, e.g. 1, 2, 3, etc.
 
+gpt_partition_bootable
+    1 if the partition is marked as bootable, 0 if not
+
 gpt swap
 ~~~~~~~~
 
@@ -167,6 +170,8 @@
     rootfs
     => echo ${gpt_partition_entry}
     2
+    => echo ${gpt_partition_bootable}
+    0
 
 Get the list of partition names on the disk::