Make sure LD -L option is calculated correctly when the project Makefile has specifie...

Make sure LD -L option is calculated correctly when the project Makefile has specified SRCDIRS components that are not directly below Makefile folder level.

For example,
SRCDIRS = comp_a  happy/comp_b  /c/dev/comp_c

Then the following are built:
build/comp_a/libcomp_a.a
build/comp_b/libcomp_b.a
build/comp_c/libcomp_c.a

But when LD is run the -L is calculated as follows
-L build/comp_a
-L build/happy/comp_b
-L build//c/dev/comp_c

This means comp_b and comp_c are not found by LD. With this change set -L is calculated correctly for comp_b and comp_c

Merges #504 https://github.com/espressif/esp-idf/pull/504