Build fails if onion-dt-overlay-sw-spi package is included (*) in
build and works fine if only package building is (M) enabled.
Onion-dt-overlay package postinst script tries to create
/sys/kernel/config/ directory which is not allowed during
image building process.
The fix introduces correct usage of IPKG_INSTROOT variable
in bash, so the postinst script should be executed only
on a target hardware.

Resolves: #91

Fix onion-dt-overlay postinst script to have error-free build

Upped package release number

Resolves: #92

Add kernel dependencies for onion-dt-overlay-sw-spi package