zipsign: Improve function to also admit single zip names
zipsign: Improve function to also admit single zip names

Change-Id: Ic5eb1ddf5a400862c859c520d43187bf0c6388f0
Signed-off-by: Adrian DC <01fb3892f39957945220431e7409156544d2f041@gmail.com>

AdrianDC/android_development_shell_tools,AdrianDC/android_development_shell_tools