scripts: Consider env variables while running 'make'

The scripts, gki/generate_defconfig.sh and kconfig/merge_config.sh
doesn't consider the host exported environment variables (such as
HOSTCC) while running 'make'. This is particularly needed in the
Android build environment as it uses the compiler, linker, and
other utilities from a pre-builts' path, rather than depending on
the host tools. Hence, pass the environment variables explicitly.

Change-Id: I082c0c05be5f4266fc647e3c1ebef8302e1a433a
Signed-off-by: Raghavendra Rao Ananta <rananta@codeaurora.org>
Signed-off-by: Neeraj Upadhyay <neeraju@codeaurora.org>