tree-wide: remove obsolete SDK in conditionals

tree-wide: remove obsolete SDK in conditionals

Obsolete use of $(SDK) in configure conditionals can result in
dependency errors when building a subset of packages for packages which
have multiple sub-packages.

The reason it causes dependency issues is that (using libdbi-drivers as
an example) lines like:

ifneq ($(SDK)$(CONFIG_PACKAGE_libdbd-sqlite3),)

always evaluate to true if you are compiling in the SDK. So for a user
compiling from the SDK, the configure arguments are always added to the package build.

In the case of libdbi-drivers:

CONFIGURE_ARGS += \
  --with-sqlite3 \
  --with-sqlite3-incdir=$(STAGING_DIR)/usr/include \
  -with-sqlite3-libdir=$(STAGING_DIR)/usr/lib

is always added even if PACKAGE_libdbd-sqlite3 is deselected. When
libdbd-sqlite3 is deselected, this dependency:

DEPENDS:=libdbi +libsqlite3

is not present, so when configure tries to find sqlite3 it fails.

Closes #28173 "tree-wide: obsolete $(SDK) in conditionals"

See also:

* "include: remove SDK exception from package install targets"
  openwrt/openwrt@28f44a4

Performed tree-wide to ease revert if necessary, per:
https://github.com/openwrt/packages/issues/28173#issuecomment-3694615980

Signed-off-by: Daniel F. Dickinson <redacted>