This had been getting parsed by
system/core/platform_tools_tool_version.mk and inserted as a version
into fastboot. Now that fastboot is converting to Soong, switch this
over to a genrule in order to extract the version number.
Test: fastboot --version
Test: inspect generated header
Test: change plat_tools_source.prop_template, rebuild fastboot
Test: change plat_tools_source.prop_template to use ${PLATFORM_SDK_VERSION}
Change-Id: Id807539ce057add50b7d10a234670820a8772351
