package/icu: adjust version information

package/icu: adjust version information

The icu upstream developers have a strange way of labelling their versions:

 - The Github tags are named release-XX-Y, and therefore the versions
   referenced by by Release Monitoring look like 60-2 or 64-1.

 - The tarballs contain the version with an underscore separator, i.e
   60_2 or 64_1.

 - The directories on http://download.icu-project.org/files/icu4c/
   contain the version with a dot separator, i.e 60.2 or 64.1.

In order to have our <pkg>_VERSION match the one on Release
Monitoring, we adjust it to use the underscore separator, and then
further tweak the ICU_SOURCE and ICU_SITE definitions to keep
everything working.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>