scripts/build-all: remove all defconfig manipulation options

Simplifiy the script by removing all options and code for manipulating
defconfigs.

Change-Id: Ic8c583d501165501035c2c43b69e5b5c57b97fb0
Signed-off-by: Bryan Huntsman <bryanh@codeaurora.org>
Signed-off-by: Naitik Bharadiya <bharad@codeaurora.org>