Change SDKPATH for something more stable in time

Change SDKPATH for something more stable in time

 * The SDKPATH must be fix to be used with YoMo.
   Inside rpm nativesdk file all the path is prefed by
   SDKPATH/sysroots/${HOSTARCH}-${SDK_VENDOR}-${OSHOST}/

 * have a (day) time stamp in ${DISTRO_VERSION} force the rebuild of
   the package every day.
eg:
At runtime /etc becomes:
(old value):
/opt/agl-sdk/6.90.0+snapshot-20180921-aarch64/sysroots/x86_64-aglsdk-linux/etc
(new value):
/opt/agl-sdk/6.90.0+snapshot-aarch64/sysroots/x86_64-aglsdk-linux/etc
in native sdk

Bug-AGL: SPEC-921
Bug-AGL: SPEC-1667

Change-Id: I28d7f8d140ba75d9d8c68c84ba5fc697e6011836
Signed-off-by: Ronan Le Martret <ronan.lemartret@iot.bzh>