kbuild-dir := $(if $(filter /%,$(src)),$(src),$(srctree)/$(src)) explain the grammar points in this sentence

This sentence is a makefile statement that sets the variable `kbuild-dir` based on some conditions.

- The variable `kbuild-dir` is being assigned a value using `:=` which is the immediate assignment operator. This means that the assignment will be evaluated immediately rather than at the time of use.

- The `if` statement is used to check for a condition. In this case, it is checking if the string `'/%'` is contained in the `$(src)` variable. If it is true, then the `$(src)` variable is assigned to `kbuild-dir`.

- If the `if` condition is not true, then the `$(srctree)/$(src)` is assigned to `kbuild-dir`.

- The `$()` syntax is used to reference makefile variables, and in this case, the variables being referenced are `src` and `srctree`.

- The backslash is used to continue the statement on the next line.

Overall, this statement is setting the `kbuild-dir` variable to either the value of `src` or the concatenation of `srctree` and `src`, depending on the condition.