lualoader: Cache kernel list

With autodetection turned on, hitting the filesystem everytime we need to
calculate choices for the kernel carousel is kind of slow. Cache once on the
first listing and reload it anytime the config is reloaded in case any of
the loader.conf(5) changes that affect this (kernel, kernels,
kernels_autodetect) have changed. This also picks up the case where we've
changed currdev and the autodetected kernels could change.

stand/lua/config.lua

@@ -497,6 +497,7 @@ function config.reload(file)
 	modules = {}
 	config.restoreEnv()
 	config.load(file)
+	core.configReloaded()
 end
 
 function config.loadelf()

stand/lua/core.lua

@@ -138,7 +138,18 @@ function core.setSafeMode(safe_mode)
 	core.sm = safe_mode
 end
 
+function core.configReloaded()
+	-- Clear the kernel cache on config changes, autodetect might have
+	-- changed or if we've switched boot environments then we could have
+	-- a new kernel set.
+	core.cached_kernels = nil
+end
+
 function core.kernelList()
+	if core.cached_kernels ~= nil then
+		return core.cached_kernels
+	end
+
 	local k = loader.getenv("kernel")
 	local v = loader.getenv("kernels")
 	local autodetect = loader.getenv("kernels_autodetect") or ""
@@ -166,7 +177,8 @@ function core.kernelList()
 	-- setting, kernels_autodetect. If it's set to 'yes', we'll add
 	-- any kernels we detect based on the criteria described.
 	if autodetect:lower() ~= "yes" then
-		return kernels
+		core.cached_kernels = kernels
+		return core.cached_kernels
 	end
 
 	-- Automatically detect other bootable kernel directories using a
@@ -195,7 +207,8 @@ function core.kernelList()
 
 		::continue::
 	end
-	return kernels
+	core.cached_kernels = kernels
+	return core.cached_kernels
 end
 
 function core.bootenvDefault()