uboot-envtools: set nonshared flag correctly per-subpackage

uboot-envtools: set nonshared flag correctly per-subpackage

Currently, uboot-envtools is being built for a (shared) instruction
set (phase2 in buildbots) instead of target-specific (phase1 in builbots).
So the package does not contain the uci-defaults file specific for each
target_subtarget. (Only the fortunate target with coincidentally the same
instruction set and target as the SDK chosen by the buildbot will have the
uci-defaults file.)

This commit sets the nonshared flag correctly in the Makefile so that
uboot-envtools is built for the target-specific (phase 1).

This is done by using the PKGFLAGS variable which is intended for
per-binary flags (instead of PKG_FLAGS which has a global scope), as
stated in some old commits 349e7b635e[1], 2d7eaf2e15[2], 064e7c8f00[3] and
2cb75cd8b9[4].

While at here, use PKG_URL, instead of URL, which is intended for global
scope. As stated in commit e32edf712b[5].

[1] https://github.com/openwrt/openwrt/commit/349e7b635ea819ad34a571bc0adcce9b205df4e9
[2] https://github.com/openwrt/openwrt/commit/2d7eaf2e15aa77bcf526b7ea9747732b35194b2e
[3] https://github.com/openwrt/openwrt/commit/064e7c8f00ebc1721553c0fcb312f809b00e179a
[4] https://github.com/openwrt/openwrt/commit/2cb75cd8b9d451bde0cfffce80fa67d58944c44b
[5] https://github.com/openwrt/openwrt/commit/e32edf712b7c43e25b4bec40fedfd948a0ed5bd6

Fixes: #19040
Fixes: 46e376c ("uboot-tools: migrate uboot-envtools to uboot-tools")
Signed-off-by: Mario Andrés Pérez <mapb_@outlook.com>
Link: https://github.com/openwrt/openwrt/pull/19180
Signed-off-by: Robert Marko <robimarko@gmail.com>