use minigzip instead of system gzip in the build

Use zlib's minigzip utility, built as part of our source tree, instead of
whatever installation of GNU gzip happens to be on the user's machine.
Using zlib's deflater, which is nicely available as a library (unlike
GNU gzip's deflater) will ultimately let us do binary patches to the
boot and recovery images.
diff --git a/core/config.mk b/core/config.mk
index 1ce5937..a5f3720 100644
--- a/core/config.mk
+++ b/core/config.mk
@@ -156,6 +156,7 @@
 ICUDATA := $(HOST_OUT_EXECUTABLES)/icudata$(HOST_EXECUTABLE_SUFFIX)
 SIGNAPK_JAR := $(HOST_OUT_JAVA_LIBRARIES)/signapk$(COMMON_JAVA_PACKAGE_SUFFIX)
 MKBOOTFS := $(HOST_OUT_EXECUTABLES)/mkbootfs$(HOST_EXECUTABLE_SUFFIX)
+MINIGZIP := $(HOST_OUT_EXECUTABLES)/minigzip$(HOST_EXECUTABLE_SUFFIX)
 MKBOOTIMG := $(HOST_OUT_EXECUTABLES)/mkbootimg$(HOST_EXECUTABLE_SUFFIX)
 MKYAFFS2 := $(HOST_OUT_EXECUTABLES)/mkyaffs2image$(HOST_EXECUTABLE_SUFFIX)
 APICHECK := $(HOST_OUT_EXECUTABLES)/apicheck$(HOST_EXECUTABLE_SUFFIX)
@@ -274,7 +275,7 @@
 # The 'current' version is whatever this source tree is.  Once the apicheck
 # tool can generate the stubs from the xml files, we'll use that to be
 # able to build back-versions.  In the meantime, 'current' is the only
-# one supported.  
+# one supported.
 #
 # sgrax     is the opposite of xargs.  It takes the list of args and puts them
 #           on each line for sort to process.