modules: lvgl: Rename the VDB custom section Kconfig name

There are 4 Kconfig names about the "Draw Buffer". Rename 'VBD' to 'VDB'
in Kconfig option 'LV_Z_*VBD*_CUSTOM_SECTION' to make name consistent.

config LV_Z_VDB_ALIGN
	int "Rending buffer alignment"

config LV_Z_VBD_CUSTOM_SECTION
	bool "Link rendering buffers to custom section"

config LV_Z_DOUBLE_VDB
	bool "Use two rendering buffers"

config LV_Z_VDB_SIZE
	int "Rendering buffer size"
	default 100 if LV_Z_FULL_REFRESH

And the draw buffer definition is now:

	static uint8_t buf0[BUFFER_SIZE]
	#ifdef CONFIG_LV_Z_VDB_CUSTOM_SECTION
		Z_GENERIC_SECTION(.lvgl_buf)
	#endif
			__aligned(CONFIG_LV_Z_VDB_ALIGN);

Signed-off-by: Haiyue Wang <haiyuewa@163.com>