loader: lua: remove the default kernel if it doesn't exist

The `kernel` env var provides the default kernel, usually "kernel".  It
may be the case that the user doesn't have a "kernel" kernel, just
"kernel.*" kernels, but have left `kernel` to the default because we
autodetect entries by default anyways.

If we're doing autodetection, take note of whether the default kernel
exists or not and remove it from the list if it doesn't and we had found
any other kernels.  We avoid it in the #kernels == 1 case because
something fishy has likely happened and we should just trust the
configuration.

Reviewed by:	imp, manu
Differential Revision:	https://reviews.freebsd.org/D42967

stand/lua/core.lua

@@ -201,17 +201,18 @@ function core.kernelList()
 		return core.cached_kernels
 	end
 
-	local k = loader.getenv("kernel")
+	local default_kernel = loader.getenv("kernel")
 	local v = loader.getenv("kernels")
 	local autodetect = loader.getenv("kernels_autodetect") or ""
 
 	local kernels = {}
 	local unique = {}
 	local i = 0
-	if k ~= nil then
+
+	if default_kernel then
 		i = i + 1
-		kernels[i] = k
-		unique[k] = true
+		kernels[i] = default_kernel
+		unique[default_kernel] = true
 	end
 
 	if v ~= nil then
@@ -239,6 +240,8 @@ function core.kernelList()
 		return core.cached_kernels
 	end
 
+	local present = {}
+
 	-- Automatically detect other bootable kernel directories using a
 	-- heuristic.  Any directory in /boot that contains an ordinary file
 	-- named "kernel" is considered eligible.
@@ -267,8 +270,25 @@ function core.kernelList()
 			unique[file] = true
 		end
 
+		present[file] = true
+
 		::continue::
 	end
+
+	-- If we found more than one kernel, prune the "kernel" specified kernel
+	-- off of the list if it wasn't found during traversal.  If we didn't
+	-- actually find any kernels, we just assume that they know what they're
+	-- doing and leave it alone.
+	if default_kernel and not present[default_kernel] and #kernels > 1 then
+		for i = 1, #kernels do
+			if i == #kernels then
+				kernels[i] = nil
+			else
+				kernels[i] = kernels[i + 1]
+			end
+		end
+	end
+
 	core.cached_kernels = kernels
 	return core.cached_kernels
 end