setup-defconfig.inc: implement config fragments by using in-kernel script
setup-defconfig.inc: implement config fragments by using in-kernel script

* Use in-kernel merge_config.sh script and KERNEL_CONFIG_FRAGMENTS list
* Move copying provided defconfig from own task to do_configure() for atomicity
* Don't use curly brackets for shell variables to avoid conflict with Bitbake

Signed-off-by: Denys Dmytriyenko <d29de71aea38aad3a87d486929cb0aad173ae612@ti.com>
Reviewed-by: Chase Maupin <874d7d4fa211492c1868f6a91442712d37040f21@ti.com>

smoosvi/Test4,tylerwhall/meta-ti,rcn-ee/meta-ti,rcn-ee/meta-ti,smoosvi/Test4,rcn-ee/meta-ti,rcn-ee/meta-ti,RedFIR/meta-ti,joelagnel/meta-ti,RedFIR/meta-ti,RedFIR/meta-ti,joelagnel/meta-ti,tylerwhall/meta-ti,RedFIR/meta-ti,tylerwhall/meta-ti,joelagnel/meta-ti,joelagnel/meta-ti,tylerwhall/meta-ti