build-scp.sh: fix packaging wrong platform binaries

The do_build() function of build-scp.sh copies the platform binaries
from ${SCP_PATH} build folder into a common output folder. If build scp
is executed for one platform (say PLAT1), and package scp is executed
for another platform (say PLAT2), instead of copying PLAT2's SCP
binaries, the package stage of build-scp.sh would copy PLAT1's binaries,
since there's no separate platform folder in the output folder of
${SCP_PATH}. To fix this, create platform folder within the output
folder of ${SCP_PATH} and copy from output/${plat}/* instead of
output/*. This would ensure that if the package is called for PLAT2, it
would copy PLAT2's binaries instead of binaries of previously build
platform.

Change-Id: I6a68e1cec9972b6dd395b9e109f7a041c15ca43b
Signed-off-by: Vijayenthiran Subramaniam <vijayenthiran.subramaniam@arm.com>