makefile: Remove uneeded vars

Signed-off-by: Ezequiel Garcia <elezegarcia@gmail.com>
makefile: Replace HOST_DIR + TARGET_NAME with single CROSS_COMPILE

Signed-off-by: Ezequiel Garcia <elezegarcia@gmail.com>
makefile: Replace PKG_CONFIG with pkg-config

If you want to cross-compile, use PKG_CONFIG_PATH
environment variable.

Signed-off-by: Ezequiel Garcia <elezegarcia@gmail.com>
makefile: Make INSTALL_DIR optional, and create the directory

This patch allows to override to default (pkg-config-obtained)
install path. Also, we need to make sure the directory is
created, for it could be inexistent.

Signed-off-by: Ezequiel Garcia <elezegarcia@gmail.com>