build: introduce PKG_BUILD_FLAGS and move PKG_IREMAP to it

build: introduce PKG_BUILD_FLAGS and move PKG_IREMAP to it

PKG_BUILD_FLAGS is a new variable for package Makefiles similar to
PKG_FLAGS. It's a whitespace separated list of flags to control various
aspects of how a package is build.

The build system and/or .config defines the default for each, but
every package has the means to override it. Using $flagname enables
a flag, no-$flagname disables it.

Start with PKG_IREMAP as "iremap". That's easy as no package here
nor in any package feed uses it. The default is unchanged: enabled.

Packages can opt-out via:
PKG_BUILD_FLAGS:=no-iremap
(Not that any should, just to illustrate how to use it)

Signed-off-by: Andre Heider <a.heider@gmail.com>