infra: support buildroot being located in /usr

infra: support buildroot being located in /usr

pkg-generic and pkg-autotools both perform sed replacements of /usr to
$(STAGING_DIR)/usr, in .pc and .la files respectively. However, if buildroot
itself is located in /usr, then STAGING_DIR also starts with /usr, and naive
replacements of '/usr' can result in paths of the form:
    /usr/buildroot/.../sysroot/usr/buildroot/.../sysroot/...

This patch makes the replacements more careful, by first replacing the known
paths BASE_DIR and STAGING_DIR to tokens, then performing the actual
replacement, and finally replacing the tokens with the corresponding paths.

Additionally it removes the unnecessary making of a backup copy in
pkg-autotools.

Partially fixes bug #5750 (https://bugs.busybox.net/show_bug.cgi?id=5750)

[Thomas: improve pkg-autotools.mk by including the explanation comment
proposed by Arnout.]

Partially-based-on-a-patch-from: Ben Stoltz <benstoltz@gmail.com>
Signed-off-by: Thomas De Schampheleire <thomas.de.schampheleire@gmail.com>
Reviewed-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>