Makefile: Pack brillo_update_payload and shflags into otatools.

brillo_update_payload and shflags are the scripts to generate A/B OTA
package (payload). Include them into otatools.zip.

brillo_update_payload was already included in previous CL. Move it to
the folder for executables.

Bug: 25715402
Change-Id: Idf9c05767c24c55d058cc30e696f06d4eb5fd53b