[meta-freescale] how to clean externalsrc?

[jags gediya]

I am building external kernel source for imx7 board.

I have done below changes for it, it is working fine while build.

inherit
externalsrc

EXTERNALSRC = /path/to/kernel/
EXTERNALSRC_BUILD = /path/to/kernel

but when i do clean through bitbake, clean is not happening in externalsrc
pointed directory. I want clean to be working on externalsrc too. what
modifications need to be done in yocto?
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